There is no hope of creating a centralized Iraq without the long-term presence of US troops - probably for decades - and the price of doing that is too high. It is morally indefensible to perpetuate this level of hardship on the Iraqi people. There seems to be only one workable exit strategy: the US must give up on the attempt to put back together what it destroyed. Iraq must be divided up - like Bosnia - into its ethnic parts. (In fact, effectively, this has already happened.)
Dividing up Iraq causes all sorts of problems. Turkey has problems with the idea of an Iraqi Kurdish state. Iran may have an easier time taking over the Shi'ite part of Iraq (and since Iran and Iraq warred over territory for decades, we can assume that Iran wants to do this). The small nation-states that Iraq will become may war. The creation of Sunni, Shia and Kurdish states may contribute to regional factionalism and instability. There is the issue of stability of oil production. And loads of other big issues.
Once we accept that Iraq cannot be put back together again, we can begin to start focusing on how to meet these challenges, and stop the unrealistic focus on how to both withdraw US troops and maintain a centralized Iraq.
The US needs to also stop the rhetoric that the problem is not getting fixed because the Iraqi congress takes a 4-week holiday or Maliki is not serious. This mess is not the fault of the Iraqis, and at this point they are largely powerless to fix it. It is not the fault of Iran or the Arab states: in fact they all officially told the US not to invade Iraq. This mess is 100% the fault of the US, and the inability to fix it is 100% the fault of Bush insisting on the unrealistic goal of creating a unified Iraq (as he did again in his presidential address last week).
As I have argued before, what is needed is a Truth & Reconciliation commission in the US, similar to what was done in South Africa after the dismantling of apartheid. The US needs to assure itself and the world that it will never commit such an atrocity again.