Here’s what I think will happen in Iraq. Power will be decentralized to the tribal and provincial level. The Kurdish provinces will remain autonomous, while the Arab Sunni and Shia regions will reach a de-facto Federal compromise. Politics is ultimately more tribal than sectarian, so there will be gradual local reconciliation rather than partition or civil war.
Each province and tribe is taking care of security and economic reconstruction. Tribes are more influencial and responsive to popular needs than the national government.
Arming each faction may or may not result in a civil war. There are signs that the three factions have already reached a relative balance of power with the Americans as the peace-brokers. The Sunnis could not destroy the Shia-Kurdish government, nor can the Shia directly impose their will on the Sunni provinces. The national government is stalemated – a reflection of the political stalemate and balance of power. And so politics and security revert to the local level.
Which is just as well, given how corrupt central governments are in the Arab world.