If the Sadrist do form an oppositional alliance, one of the key common elements will be a demand that US forces leave. The timing and the completeness of the leaving will be up for negotiations, but as Ian Welsh pointed out at the Agonist, it will be a demand in line with Iraqi public opinion:
one of the points Matt makes is that Iraqis don't want American troops in Iraq. With the exception of a conversation about Kurdistan (leave troops there, they want us) most of the commenters ignore this point. They talk about keeping "flexibility" to keep troops in country, or to keep them just outside and pop in and out every once in a while.
The Sadrists and a decent chunk of the Sunni-Arab community that is represented in the government, which is a fairly significant qualification, are vehemently anti-US. Non-represented militias, and insurgent groups are even more vehemently anti-US and these are the groups that are in line with Iraqi public opinion within the contested zones, and they have credible loyal military options. As US forces withdraw, these are the groups that have the capability to cause mass disorder for the Iraqi government OR a reasonable measure of civic peace. They will get what they want, and that means the United States needs to realize that we will continue to not get what what we want.
We will not be able to maintain permanent bases. We will not be able to slip an airborne brigade into Haditha for a week and then fly it back out to Kuwait or Irbil or Incrilik without local opposition. We will not be able to run the Iraqi Army's logistic and depot level maintenance programs. We will not be able to have a couple of brigades garrisoned at Baghdad International Airport or Balad without nightly mortar attacks and daily ambushes of any ground supply convoys. When the United States leaves, the leaving will be near complete.