Do you really think Iran is going to attack the US? That would be instant suicide and they know it. They aren’t equipped like Russia or China, you know.
But do I think Iran is likely to attack us directly? Probably not. However, you're neglecting to consider another possibility -- Iran using the various terrorist orgs it sponsors to do its dirty work. They could smuggle a nuke into one of our harbors. ...or across out porous borders (no thanks to Bush).
Nukes floating around the Islamic world is a completely unacceptable state of affairs to all rational people. It's dangerous enough that Pakistan has a nuke arsenal. ...and that alone could come back to haunt us. Toss Iran and the rest of the big Islamic players in to the nuclear mix and the situation becomes exponentially more hairy.