Shia eventually kill or drive out all of the Sunnis. That seems unlikely, particularly in Anbar province which is mostly desert. The Kurds cannot declare independence without antagonizing Turkey, but they want to continue to be almost completely autonomous. So they will want to maintain the appearance of a federated Iraq.
Some Sunnis (not the militants of course), want the US military to stay, if only to keep the Shia from outgunning them with the army and police all controlled by the Shia. Even though a federation might lead to even more migration of people away from mixed areas into "pure" ones, it almost seems inevitable now.
There was a commentary in the Dayton Daily News today by Don Hays, R. Bruce Hitchner and Edward Pl Joseph who have experience with Bosnia.
http://www.iht.com/articles/2007/01/19/opinion/edjoseph.php"...the Dayton Agreement, which just marked its 11th anniversary, ended Bosnia's bitter war (which remains, per capita, more lethal than that of Iraq.)" "But after 11 years of intensive international effort, it is time to face up to the sad reality. Bosnia's Serbs, Croats and Muslims simply do not share a common vision for the country."
The Serbs and Muslims are farther apart than ever as far as wanting to jointly govern a unified Bosnia. The Bosnian Serbs are threatening to secede and join Serbia proper.
I am not sure what the Bosnian example has to say about Iraq other than a federation may be a tough thing to make work, if the parties don't want to work with each other. Perhaps it would just be a stepping stone to each group having its own independent country. Yugoslavia broke up and life goes on. I suppose if Iraq breaks up, life will go on as well.