ArabicNews.com contains a long report concerning the possibility of an Iraqi civil war:
Revenge assassination raises fears of sectarianism
...reciprocated assassination operations took place between the Sunni and the Shiite, and raised fears of revenge operations in various parts of Iraq between the supporters of the former regime and its opponents.
The Washington Post considers it to be an overdue rash of revenge killings against Baathists:
Iraqis Exact Revenge on Baathists
The massive settling of scores that some U.S. and Iraqi officials had predicted did not initially materialize after Hussein's government fell in April. Sporadic killings occurred during the following months, notably in the southern city of Basra. But only in recent weeks did the tempo of attacks accelerate as Iraqis, frustrated with the slow progress of the court system and fearing that Baathists may be seeking to reorganize, have increasingly taken justice into their own hands, according to Iraqi security and political sources.
I don't have any insight, I only offer this for your consideration. The first article doesn't describe the target of Shiite violence as necessarily Baathist, but Sunni in general. (Iraq's Shiite majority was ruled by the essentially Sunni Baathists.) If this reciprocated intra-Iraqi vigilantism spreads beyond Baathist retaliations, or if Shiite leaders endorse such violence, it seems as though Iraq could quickly devolve much further than we've yet witnessed.