The Pithlord would like to point his readers (now securely in the daily double digits -- thanks Slate!) to an article from In These Times about the difficulties of US withdrawal from Iraq.
Total withdrawal isn't really an option, but phased withdrawal is beginning to look inevitable. Clearly, a civil war, in absolute terms already as bloody as Lebanon's between 1975 and 2000, is already under way. The author is therefore rightly nervous, and claims the Shi'ite parties want the Americans to stay, but it is clear that the US troops are providing no real source of security.
The best suggestion is from Zbigniew Brzezinski, Carter's (relatively hardline) National Security Adviser. He thinks the government should loudly ask the Americans to leave, and negotiate a phased withdrawal. This will give them some legitimacy with the Sunnis. Obviously, this would have to go along with offers of power-sharing with some of the Sunni militias.
In These Times argues that the sectarianization of Iraqi politics is a result of bad American policy mistakes. That is possible, but it is hard to see how else a foreign occupier -- basically uninterested in the local language and culture -- could otherwise understand and deal with a post-totalitarian society. The basic problem was foreseeable in 2003.
The best the US can do now is gradually extricate itself from the country. And then run every neocon/neolib pundit responsible for this out of town on a rail.