Juan Cole has an interesting take on Fallon's resignation.
Secretary of Defense Robert Gates denied Tuesday that the abrupt resignation of Admiral William Fallon as CENTCOM commander indicates an imminent war against Iran. I think Gates's denial is credible. There is no sign of an American war on Iran, which would involve key positioning of warships, materiel and troops. There is no congressional mandate for such a thing, despite the non-binding Kyl-Lieberman resolution in the senate. A provocation is not out of the question, but it would be a risky move in an election year and could easily backfire on the Republican Party (ask Aznar in Spain).
My guess is that the real reason for moving Fallon out is not Iran but Iraq....
Having such a big dissenter as CENTCOM commander is inconvenient for the Republican Party at a time when John McCain is admitting that if he fails to convince the American people that the surge is succeeding, he will lose the presidency. That is, Fallon may have run afoul not of Cheney on Iran but McCain on Iraq. This may be Bush's first favor to the Republican nominee, who after all had a career as a naval officer himself.
This is very reassuring to read and Cole knows the situation far, far better than I, of course. But I would feel more at ease if I didn't suspect that the White House has already written off the election, and had already declared that whatever the president did was legal.