Noam Chomsky: My assumption all along has been that they would not attack Iran, and I strongly suspect that the Pentagon and US intelligence (and their British counterparts) are trying to leak dire warnings in the hope of cutting it off at the pass.
I suspect that's the basis for what Seymour Hersh recently reported, and other reports like it at about the same time. You might be interested in an article in the Atlantic Monthly, Dec. 2004 (online), a wargame about attacking Iran, which has quite an authentic feel to it. Participants were high level, and genuine. The final conclusion after running through many options (including several variants of what you mention) was expressed by General Gardiner, who runs Pentagon war games of this kind:
"After all this effort, I am left with two simple sentences for policymakers," Sam Gardiner said of his exercise. "You have no military solution for the issues of Iran. And you have to make diplomacy work."
But that doesn't mean that Cheney-Rumsfeld-Bush will follow that advice. They have created a major catastrophe in Iraq, not only for Iraqis but even for themselves, and they may be desperate to prevent what could turn into a real nightmare for them. A man-eating tiger in your backyard is not very pleasant. A wounded one is a lot worse.