From the WSJ editorial page, make of that what you will, but they assert that Admiral Fallon did not resign because he was against invading Iran, but he was against continuing to invade Iraq. Or something like that:
A fateful debate is now taking place at the Pentagon that will determine the pace of U.S. military withdrawals for what remains of President Bush’s term. Senior Pentagon officials — including, we hear, Defense Secretary Robert Gates, Joint Chiefs Chairman Mike Mullen, Army Chief of Staff George Casey and Admiral Fallon — have been urging deeper troop cuts in Iraq beyond the five “surge” combat brigades already scheduled for redeployment this summer.
[snip]
This Pentagon pressure also does little to help General Petraeus. The general is supposed to be fighting a frontal war against Islamist militants, not a rearguard action with Pentagon officials. We understand there is a chain of command in the military, and General Petraeus is precisely the kind of team player who would respect it. That’s why as Commander in Chief, Mr. Bush has a particular obligation to engage in this Pentagon debate so that General Petraeus can make his troop recommendations based on the facts in Iraq, not on pressure from Washington.