CNN
Blackwater employees were involved in a gun battle in Baghdad that resulted in 8 civilian deaths. The Iraqi government has revoked the license of the mercenary army employed Blackwater to do business in Iraq and Prime Minister Malaki has labeled the incident a "crime". Condoleeza Rice is carrying water for Blackwater and is working on smoothing things over with the Iraqis.
I'm starting to think that barring a Democratic presidential victory, the most likely scenario for withdrawl is for the Iraqi government to say we're no longer welcome. This would probably involved the Maliki government being deposed in favor of someone like Sadr. Dare to dream I guess.