My own hope is the the Project for the New American Century policy wonks have been pushed back into an annexe by the inner circle in the White House, and Bush's primary concern now (like Blair's) is his legacy - he's concerned at how he'll be perceived by posterity and doesn't have the nerve to start any more wars. Hence the recent shuffling of the cards in the cabinet.
Thus I don't think he will actually attack Iran. Plenty of sabres will be rattled, but I think the hope is that Ahmedinajad will tacitly back down before push comes to shove- and then Bush will claim that as a triumph for his special brand of diplomacy.
Militarily the US hasn't got what it takes to carry out an invasion of Iran. A few Tomahawk strikes, yes, but anything dustier, forget it! This is Bush as poker player - but the trouble is, Ahmedinajad is as aware of his weaknesses and unpopularity as anyone else. It's not a good situation in which to be player poker, particularly given the stakes.