Mahdist movements throughout history have been characterized by dreams presaging the alleged Mahdi's appearance, whether those of the Mahdi claimant himself, his followers, or both.  Such was the case with the two most prominent Mahdist movements in modern times, those of the Sudanese Mahdi Muhammad Ahmad in the 1880s and the Saudi Mahdi Muhammad Abd al-Qahtani in 1979.  Usually, however, such dreams instruct folks to kill establishment opponents of the Mahdi, rather than family members:

Dad said Imam Mahdi ordered him to kill his family
Kuwait's 'dream killer' must hang: court
Kuwait's appeals court on Monday upheld a death sentence handed down against a Shiite Muslim who said he had been ordered in a dream to kill his wife and two children.
Dhaher al-Fadhli, 47, shot and killed his 38-year-old wife Badriya, his eldest son Waleed, 21, and 16-year-old daughter Baida in December 2004, claiming he was ordered to do so in a dream by Imam al-Mahdi, the "hidden imam" and a central figure in the Shiite faith.... (http://www.alarabiya.net/articles/2007/11/27/42191.html ).

Note that this article editorializes, wrongly, that the Mahdi is only a Shi`i belief.

Another new Mahdist movement has developed in southern Iraq.  Back in January of this year Jund al-Sama' ("Army of Heaven"), led by the self-styled Mahdi Dhiya Abd al-Zahra al-Gar'awi, vainly fought against American and Iraqi government troops.  Now one Ahmad al-Hassan is claiming to be "the son and herald of the Mahdi," according to this anti-Shi`i Muslim site: http://antichrist-and-shiism.blogspot.com/2007/11/messianic-shia-cult-emerges-in-southern.html
"Very little is known about al-Hassan. He moved to Najaf to receive religious training after he received his Bachelor’s degree in civil engineering from Basrah University during the late nineties. He immediately collided with senior ayatollahs when he called for reforms in the religious seminary, which he described as being rife with financial corruption and mediocre scholastic curricula, earning him the backing of disgruntled clerics and students....He remained under the radar, but his followers said he was placed under house arrest by the Iraqi government in Basrah last year, and many of his followers have been detained in several southern cities....Ansar al-Imam al-Mahdi is just one of several Shi’ite millenarian movements that have proliferated in southern Iraq, such as that of Ayatollah Mahmud al-Sarkhi in Karbala whose followers have been detained by local Iraqi troops loyal to the Supreme Council of Islamic Revolution in Iraq (SCIRI), Iraq’s leading Shi’ite political party. Groups that preach the imminent return of the Mahdi are called Mahdawiya, and the clerical establishment in Najaf, headed by Grand Ayatollah Ali al-Sistani, does not look favorably on them. Most are influenced by the teachings of the late Grand Ayatollah Mohammed Sadiq al-Sadr, the father of Muqtada al-Sadr....Sources close to the office of Grand Ayatollah Kazem al-Haeri in Basrah said that he has issued a fatwa authorizing the killing of al-Hassan if he does not recant his claims. Sistani’s office in Najaf distributed fliers two months ago warning Shi’ite pilgrims from imposters claiming to be “messengers of the Imam Mahdi.” Two other senior Shi’ite ayatollahs, Sheikh Bashir al-Najafi and Sheikh Ishaq al-Fayyadh, also released statements stating that anyone declaring representation of the awaited Imam is a `slanderous liar'....The Ansar are not known to have taken up arms yet.... Al-Hassan said he has ordered his followers to lay low and move to other parts of the country to avoid a clash with authorities....al-Hassan’s movement has attracted several thousand followers in Iraq, some of them from the opposite Sunni sect, and even some Christians. Al-Hassan claims to have followers in Iran, Lebanon, the Gulf, Pakistan, Egypt, Morocco, and even among Shi’ite communities in Europe and North America, who help fund the group through donations. The group’s website on the Internet (www.almahdyoon.org) is gaining increased attention...."
Observations:
1) Mahdism would seem to be growing in power in the Islamic world, if Sunni websites now deem it necessary to begin mentioning it.
2) Mahdism has more forms than just the institutionalized one of the Islamic Republic of Iran.
3) The issuing of  anti-Mahdist fatwas follows in the historical traditions--which I trace in my book Holiest Wars--of the first phase of Islamic establishments' (both political and religious) attempts to de-legitimize Mahdist criticism (the second phase is usually a military response).
4) Note the pan-Islamic ecumenical claims of the Ansar al-Imam al-Mahdi.  Also, if you look at their website, it's clearly designed to draw in not only Sunni Muslims but also Christians!

Kent Brockman: The weather service has warned us to brace ourselves for the onslaught of Hurricane Barbara. And if you think naming a destructive storm after a woman is sexist, you obviously have never seen the gals grabbing for items at a clearance sale.
Marge: Hrm...that's true... but he shouldn't say it.
This past Saturday Senator John McCain played Kent Brockman and a Dartmouth student played Marge Simpson: "At a town hall-style meeting at Dartmouth College in Hanover, McCain was challenged by a student over some of his remarks about Muslim beliefs. "The world's oil supply lies in the hands of some very unpleasant people: Mr. Chavez down in Venezuela; Putin in Russia; the president of Iran, who as you know continues to espouse the extermination of Israel and some whacky theory about the 13th Imam and that you've got to have Armageddon or something. I still haven't figured that one out exactly," McCain said. Dartmouth freshman Neel Joshi of Los Angeles asked McCain: "Do you feel failed cultural understanding led to our failed policy in Iraq and if so, do you believe that comments you made at the beginning of your speech here, about the 12th Imam — and ideas of Shiia Muslims as crazy — is contributing to that problem and will alienate not only in this country but also in the entire Middle East?" McCain said the failure in Iraq is based on policy, not religion. "I don't claim to be an expert on the Muslim religion ... but everyone I know that is an expert on the Muslim faith says the president of Iran's theory is one that is not shared by the overwhelming majority of people of that faith, that there has to be an Armageddon and the 12th or 13th — I thought it was the 13th — Imam comes to power," McCain said. "I'd find that very disturbing if that view was shared by a lot of people." After the event, Joshi said such statements undercut the United States and hinder hope for international cooperation. McCain later told The Associated Press he didn't mean to sound harsh, but that he has trouble believing Armageddon must come for a new leader to emerge. "It is dangerous to have a leader of a nation who believes and desires and promotes Armageddon to advance a particular religious outcome," he said" (complete story: http://www.newsmax.com/insidecover/mccain_clinton_respect/2007/11/18/50422.html).

Observations:
1) By the time you've made it to Dartmouth, shouldn't you be able to make subjects and verbs agree?
2) Why did the AP writer who filed this story deem it necessary to find Joshi afterwards and elicit his opinion on international affairs--especially considering he can't make subjects and verbs agree?  Somehow I think that college undergrads who ask difficult questions to, say, Hillary Clinton or Barack Obama are not given the same opportunity to expand on their comments afterwards.
3) Those shots aside, Joshi did have a valid point, albeit no-doubt unintended: there is a Western (particularly secularist) tendency to make the ideas in Shi`i Islam about the return of the Twelfth (not Thirteenth, Senator) Imam appear crazy. Along with 2.3 billion other people who recite the Apostles' or Nicene Creeds every Sunday, I happen to believe in  the eventual return of a fellow who was crucified two millennia ago--which is pretty "whacky" to the likes of, say, Christopher Hitchens.  It's not belief in the supernatural return of a messianic figure per se that is "whacky;" rather, it's the idea that that said messianic figure will come only after a conflagration, possibly a nuclear one, has been sparked.
4) And on that topic of the alleged need in Shi`i Islam for "Armageddon" to occur before the Twelfth Imam will return as the Mahdi: McCain, no doubt following briefings from his advisors, repeats this charge--but as I've said numerous times, I am not convinced this is actually a tenet of Twelver (or Fiver, or Sevener) Shi`ism. Neither has it been proved, despite numerous claims, that the Hojjatiyeh organization--the primary mission of which is to combat (and indeed persecute) Bahai's--holds the belief in the need for mass destruction to hasten the Mahdi's appearance. 
5) Joshi's comments, and reporting on them in this AP article, show once again that the mainstream media regards exposing potentially dangerous Islamic ideas as more threatening than the ideas themselves.  Ditto for jihad: as I've encountered numerous times when writing or speaking on jihad in its primary historical and theological sense of holy war, the American media and professoriate simply can't handle the truth.

Stephen Schwartz has reviewed my first book (the second one, on the caliphate, is still being researched ) in Middle East Quarterly: http://www.meforum.org/article/1778 .
I appreciate the overall positive take; and I totally agree with Mr. Schwartz that  "a serious, detailed, and comprehensible look at Shi‘i beliefs about the twelfth imam and his occultation and how they play out in the speeches of Ahmadinejad as well as the armed intrigues and depredations of the Sadrists, would be extremely useful to Westerners right now."  But so far Ahmadinejad has yet to put his money where his Mahdist mouth is--whereas Sunni Mahdis have cropped up as far afield as Bangladesh, India, Gaza and Indonesia just this year alone. Plus, there's that little matter of that violent, Shi`i-Sunni combination, Soldiers of Heaven Mahdist movement that erupted in southern Iraq in January 2007.  I'm not sure Mr. Schwartz is right that Shi`i Mahdism is more threatening, or important, than the Sunni brand.

Recent news stories have reported that one major reason for the decrease in violence in Iraq--besides the surge and General Petraeus' new strategy--is that the Shi`i Jaysh al-Mahdi has been cooperating with U.S. and Iraqi government forces against "rogue" Shi`i militias.  But according to Newsweek, the reason that the Army of the Mahdi is working with us infidels may simply be that its commander, Moqtada al-Sadr, is studying for final exams:
"U.S. commanders think the 36-year-old cleric has temporarily relocated to Iran. But a source in the Shiite holy city of Najaf who also asked to remain anonymous says Sadr’s gone underground there. He claims that Sadr is cracking the books, hoping to elevate himself to the level of hojat olIslam—one step below ayatollah. Some in the Shiite howza, the clerical elite that surrounds Grand Ayatollah Ali Sistani, scoff at the attempt. “His mentality does not allow him to reach higher levels of study,” says one high-ranking howza scholar. But Sadr’s instructors are thought to be followers of his assassinated father, Ayatollah Mohammed Sadiq al-Sadr, and they might be inclined toward grade inflation" (http://iraqupdate.wordpress.com/2007/11/13/al-sadr-hits-the-holy-books-while-mahdi-army-ceasefire-continues-in-iraq/).
Just what Iraq needs: a high-ranking Shi`i cleric who believes the U.S. invaded Iraq to preempt the Mahdi's emergence.  Heretofore the Mahdist firebrand senior religious figures were in Iran.  Now we may get one in Iraq.  If al-Sadr passes his exams--and who would be foolhardy enough to flunk him, with his armed study group?--look for the Army of  the Mahdi to renege on its cooperation with the Great Satan soon after.

2007 is turning out to be the Year of the Mahdi(s): yet another has emerged, this time in Indonesia.  And his followers have engaged in gun battles with police, killing several on both sides.  Din Syamsuddin, head of the Muhammidyah, has (of course) labeled the Mahdists "blasphemous" and painted them all as uneducated, isolated and poor (http://wwrn.org/article.php?idd=19304&sec=55&con=19).
At the same  time, also in Indonesia, one Ahmad Moshaddeq, a.k.a. Abdussalam, has "declared himself a new prophet, replacing Prophet Muhammad, on July 3" (http://wwrn.org/article.php?idd=26772&sec=55&con=19).  His al-Qiyadah group has existed since 2000, and is said to have 40,000 followers.  Interestingly, prophet Moshaddeq has been arrested for violating Indonesia's shari`ah-based blasphemy law.
Observations:
1) Worldwide Relgious News, which carried these stories, lumps both these movements under the rubric of "Other NRMs"--"New Religious Movements," that is.  While this is accurate in the case of someone declaring himself a new prophet in Islam, it is misleading in the case of someone declaring himself the Mahdi.  Mahdism is quintessentially Islamic, for according to Islamic traditions the Mahdi will refurbish and reinstitute the original Prophetic teaching and practice, not bring a new dispensation.
2) Note that the new Indonesian prophet's movement is not violent (yet), whereas that of the self-proclaimed Mahdi is.  Anyone in Washington listening?
3) Religious leaders in Indonesia are already stereotyping the Mahdists as poor, dumb and probably in-bred.  This is a variation on the theme of "poverty produces terrorism," and it's just as wrong about Mahdists as it is about jihadists.  Historically, Mahdist claimants and their followers have often been educated and well-off, and while they have indeed tended to come from the geographical, sociopolitical or intellectual margins of Muslim societies that does not mean they were social misfits. 
4) How moderately-Muslim Indonesia deals with such movements--Mahdist and heretical--will prove instructive as the U.S. starts to encounter more and more such movements in Iraq and elsewhere in the coming years.