One aspect of the possible Iraqi civil war that rarely gets mentioned is that there's no good reason to assume it will involve only Iraq. There are Sunni and Shia Muslims spread throughout the region. Would either one of those groups be willing to sit back and see how things play out in Iraq? During her recent tour of the Middle East Condoleezza Rice was told that sectarian violence could spread throughout the region (& beyond? - what about Pakistan?).
I've heard a few people compare Iraq with Lebanon. However, what rarely gets mentioned is that Lebanon was a Christian vs Muslim battle - the Lebanese Christians didn't have a large sympathetic population in neighboring states.
I'm not quite ready to throw in the towel on the Iraqis yet. I still believe a national unity government is possible and that common sense will keep the bulk of Iraqis working towards a peaceful and prosperous future. But, that scenario sure doesn't look inevitable this week.