[Watser?]

The question is good, I just don't think the answer is. I can see how the Kurds can secede, but even there the problem of Kirkuk is a major one. Neither side can live with signing it away in any agreement. And even if Arabs and Kurds agree on it, there is still the problem of the Turkmen. They are like the Sudeten Germans of Iraq: the Turks will intervene 'to protect their brothers'.

As for the Shi'ites and Sunnis not being able to live together: the Lebanese have a pretty bloody history too, the Tutsis and Hutus are still living together in Burundi and Rwanda. The outcome is still far from clear.

The thing is the Sunnis will never go along with partition because they are not being offered a viable state, so the proposed solution of partition is not a solution but a new problem. Plus the Mahdi Army is very much opposed to it as well and they have already been having violent clashes over it with SCIRI (Supreme Council for the Islamic Revolution in Iraq). The Mahdi Army is very much Iraqi nationalist and see SCIRI as Iranian agents (and they have a point). They are not gonna go along with partition. I would not even be surprised to see a new coalition between the Sunni and Shi'ite nationalists opposing partition.

Another problem of course is the US will be seen as the evil outside power breaking up Iraq to divide and rule.
Either way, I think the US presence is causing more violence than it is preventing. There is no point in staying, unless the US wants to be an empire.