Although I'm on record as predicting the endurance of the Ayatollahs' regime beyond the end of the year, nobody would be more pleased than I to be proven wrong in this case. Nevertheless, I note that so far the anti-government demonstrations in Iran seem to lack three crucial criteria for success:

Mass support. Crowds have been reported in the hundreds or thousands, not hundreds of thousands. The regime has been enduring that sort of minor irritation for years now.

Signs of weakness in the regime. The closest the government has come to capitulating to its opponents is its arrest of some of its own thugs, after they rampaged through some universities beating up anti-government students. This could easily be a purely symbolic gesture meant for foreign consumption, rather than a genuine concession to the opposition. And otherwise, the evidence of cracks in the Ayatollahs' power structure is pretty scarce. (Kieran Healy elaborates on this point, adding some scholarly detail.)

An iconic leader. I don't recall a single popular uprising against an indigenous tyrant that has succeeded in recent years without first rallying around a single opposition leadership figure. Nor am I aware of such a figure emerging in Iran. (Of course, I urge any reader with the knowledge to correct me on either of these points to post a comment with details.)

I don't doubt that the Iranian clerical regime is deeply unpopular, that its opposition has been strengthened by the fall of Saddam Hussein, and that its longevity is in doubt. But I also suspect that a great deal more will have to happen before it progresses from just another hated dictatorship to another notch on the belt of democracy. The time scale of that progress is likely to be a lot longer than the Michael Ledeens of the world tend to imply.