Ossobuco:
I'm more worried that Israel will do the pre-empting rather than us. From a purely military perspective, Iran presents a more viable "Clear and Present Danger" to them. Despite all the hype about Israel being our surrogate, they have proven over the years to be quite independant in their actions when they feel threatened, even to the extent of attacking one of our intelligence ships in '67. (Yeah, I know, it was "an accident". At least that is the "official" story.)
Frankly, the fact that Iran might acquire nukes, while of serious concern, is not nearly the "Clear and Present Danger" we make it out to be. At least to us. Any country in the world with nukes knows that if they hit us, our retaliation would be swift and deadly. While target theory was not an area I dealt with during my Intelligence career, I expect Iran could be set back some 400 years with the payload of one stealth bomber, or one ship's worth of Tomahawks, or any of the other wonderous means we have to deliver "The Wrath" with. I can think of no country's leadership who is so crazy as to desire the complete destruction of his/her country.
The problem is going to be Israel. If, say, Iran hits Israel. Will our Government honor our commitments to that country? Or will we dance while "Rome burns"? I wonder if Israel is wondering too.......

Our steadfastness in "commitments" to other countries has been somewhat spotty since '72 or so.
Halfback