The Shi'a spent the night lobbing mortars back and forth at each other in the northern Baghdad neighborhoods, dropping a couple on the Green Zone as well just for good measure. Its only a matter of time before the U.S. military deploys a Predator drone in a counter-battery role to start striking back at the insurgent mortar positions. Traditional counter-battery operations are difficult because the mortars are being fired from within built-up areas. The Predators can loiter over the city for hours on end at night and easily identify the firing point hot spots. Then it's a simple matter of a very fast and accurate response with a Hellfire missile.
The situation has devolved away from Muslim vs Westerner, past Shi'a vs Sunni, to now Shi'a vs Shi'a. The Mahdi Army just can't seem to get along with anyone right now.