Take a look at this from the article by Andrew Cockburn over at Counterpunch
Last week, Tehran reminded us of its ability to make life unpleasant for US forces in Iraq by hosting Moqtada al Sadr for a high profile visit, in the course of which he obligingly pledged that his militia, the Mahdi army, would retaliate for any American attack on Iran. His spokesman quoted him as telling his hosts "If any Islamic state, especially the Islamic Republic of Iran, is attacked, the Mahdi Army would fight inside and outside Iraq."
Okay, so if the United States is foolish enought to attack Iran, Al Sadr would either join the Iranian forces against our troops in Iran, or, in the case that Bush and his merry band of elves decided to us just air power against Iran, Sadr would order his troops (militia) to attack US troops in Iraq...
I read that pretty much as a lose/lose proposition for our side.
Also...do you consider it odd that the meeting between Al Sadr and the Iranian government went unreported in the U. S. Press? That mainstream media didn't say much either.