Let's say the U.S. does nothing.
Sure, why not start off with the boilerplate false dilemma. As if the only two choices were nothing or unilateral and unprovoked military attack.
I would think there is a good possibility that Israel would attempt something, before there is an Iranian bomb. That could devolve into an international situation reminiscent of the 1914 Archduke dilemma.
So, to avoid the possibility that Israel might act in such a manner as to put its existence in further peril, the US ought to go ahead and do the deed. Grand idea. The Arab/Muslim world (not to mention the rest of the world) will understand that this was a US act and therefore hold nothing against Israel, giving it a friendly wave as the thousands of newly radicalized muslims set off to attack the US.
I am having trouble believing that even this administration and this military will carry through with an attack on Iran. I doubt that any western nation, with the possible exception of Howard in Australia, will give even a nod of support. Anti-american sentiment worldwide will increase even further. Terrorist recruitment will be facilitated by some unknown but significant factor. The US military, already stretched thin, will not be off the hook even if the attack is air and missle and coastal bombardment because Iran will not merely sit back and say 'ouch'. And the consequences for the Republican party and the new conservative movement, with which this administration is solidly identified, will be profound.
The neoconservative crowd has not stopped pushing for an attack on Iran since the mid 90s. Even though they have lost credibility with pretty much everyone other than the Fox-attending Bush base and the worst of the never-question-Israel radicals, it is unknown now to what degree their deep failures regarding Iraq have diminished their actual power in the administration and the pentagon. We don't see much of Perle or Wolfowitz any longer but many others are still highly visible and continuing the warmongering patter.