Do a blog search and you'll see some of the chatter beginning.
Straight line to the NYTimes, and it's a lot less clear:
The incident took place on Sunday in Nisour Square, an area in western Baghdad that is clogged with construction and concrete blocks. American officials said that a convoy of State Department vehicles came under fire, causing one to break down. It was towed. The officials did not say whether any of the convoy’s security guards fired back or whether they worked for Blackwater.
My two cents? Blackwater stays put. The State Department needs them to much (diplomacy is not an effective deterrent against complex ambushes). The spin? A car bomb, some gunfire and then a statement from the Interior Ministry? Sounds too pat. Probably just Jaish al-Mahdi acting up.
You now how those kids like to play.