Douglas Muir has a very interesting post running down some of the political science on “protestor versus regime” scenarios. Unfortunately, it ends on a somewhat depressing note:
And finally, the government is both willing and able to use massive force: China, Burma, Armenia. In these cases, the government wins. There is, in recent history, not a single clear counterexample. If the government keeps its nerve, and the men with guns stay loyal, and the regime is willing to escalate without limit — the government wins.
Relevance to Iran: Looks pretty high right now. While there are some reports of unease among the security forces, it appears the police and the military are holding steady.
Until and unless this changes, Ahmadinejad looks quite secure — green paint and massive street protests notwithstanding.
One hopes this will prove wrong.