is the iranian regime vulnerable?

Douglas Muir has a pretty good breakdown of exactly what makes a regime vulnerable to collapse – and more importantly, factors that make collapse unlikely.  His verdict regarding Iran?  The regime holds steady unless the guys with the guns change sides:

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.