What I think (I don't know if I am right), but since 9-11 Iran's hand has been considerably strengthened. Iraq to the West, its former enemy in war is vanquished and a naturally sympathetic Shia community has risen in stature as a result. To the east, the Taliban has fallen and Pakistan has little ability to project power which makes the Iran the biggest regional and most stable player.
Because American resources are tied down in Iraq, there is very little to check any Iranian moves in the region that are not overtly hostile (building a bomb is something the Iranian people apparently want).
As I see it, Iran has the next 5-10 years to go without a true regional adversary or the ability or willingness of the world to check it in any way. They have managed to stay out of the Iraqi post-invasion, because what we are proposing to make is much better than what they had to live with previously.
In poker terms, the U.S. has busted the other players at the table but doesn't have the stack to take Iran out, so Iran has reaped a dividend from US actions in the past 4 years. The caviar is tasting better than ever in Tehran.