Most Shia Arabs living in Baghdad have shifted in recent months from preferring the open-ended deployment of foreign troops in Iraq to wanting a one-year timetable for withdrawal. Nonetheless, a growing majority of Shias in the conflict-ridden capital say that if U.S.-led forces leave within six months there could be an upsurge in inter-ethnic violence.
So, there are two points here: (1) Shia Arabs in Baghdad want the U.S. out pretty soon, and (2) they worry at the same time that this could lead to more violence. Shias outside of Iraq agreed on the first, but were more likely to think that violence would decline if the U.S. left.
I think there's a good argument that U.S. troops are a big cause of the current violence. It is risky, though, to pull out in order to test that hypothesis.