The Kurds are making a land grab, Sistani wants a theocracy, and the Sunnis won't be happy as a relatively-powerless minority faction. When Sistani decides he wants us to go, we'll have to go. Odds are good civil war will occur soon afterwards.

From a Washington Post.com article:

Othman, the KDP leader in Irbil, called Sistani's opposition to federalism, as well as to Governing Council proposals to choose a new government through caucuses, a "dangerous hint" of things to come. "Rejecting a Governing Council decision is like rejecting the rule of civil law," Othman said. "He wants to impose a kind of religious rule on Iraq. Kurds don't accept that. Our parties are secular."

If someone like Sistani runs Iraq, "it will be worse than Saddam. No more dictators for us," said Ali Abdullah, a barber.

http://www.washingtonpost.com/wp-dyn/articles/A61702-2004Jan29_2.html