Juan Cole is skeptical of reports that he's fled to Iran. Mark Steyn and Michael Ledeen are certain that Muqtada's in Iran, and claim this as proof that the surge is working.
Oh man, that's a tough one. Who to believe? On one hand, you've got a guy who's been pretty much right about everything having to do with Iraq and Iran over the last four years. On the other, you have Mark Steyn and Michael "Islam is very unpopular in Iran nowadays" Ledeen, whose incredible ability to be wrong about every. last. thing. relating to Iraq and Iran has been matched only by their ability to soldier on without ever seeming to notice.
As Cole notes, it's in Muqtada's political rivals' interest to promote the idea that he has fled to Iran, as Muqtada has frequently condemned leaders who chose exile during the Saddam era, rather than stay in Iraq to struggle and risk death as the Sadrs did. This has proven to be a very effective rhetorical weapon for Muqtada against better-funded rivals like the Hakims.
Labeling Sadr "a Persian stooge" and "an Iranian agent," Steyn and Ledeen seem unable to grasp the reality that Muqtada represents a genuine political movement in Iraq, one rooted in a tradition of clerical activism going back decades. They hide information like this in places called books.