Look at it from their perspective. They've got their own internal unrest to worry about, and a lot of Iranians may well be looking across the border wondering whether we might help them out, too.
Not to mention that W named Iran as one of the Axis of Evil -- and Iraq is living proof of what happens to Axis-dwellers. They'd probably be uncomfortable having a potential invasion force poised on their borders.
And, come to think of it, a look at the map (caution: big) shows that Iran has reason to be worried about that. We're in Iraq with big forces on the west, and Afghanistan with big forces on the east. They cannot avoid the conclusion that they're caught in a vise -- and I can't avoid it, either.
Not to mention that W named Iran as one of the Axis of Evil -- and Iraq is living proof of what happens to Axis-dwellers. They'd probably be uncomfortable having a potential invasion force poised on their borders.
Welllll, as long as we are in the neighborhood, anyway....