This is bad. It's good that they are working together instead of killing each other for now, but not so good that they are working together to kill us. Also, I'm sure if we leave they will stick together long enough to exterminate the kurds before they turn on each other.
Most accounts on the ground that I've read maintain that the militant Shiites are in the minority and most still want to try and make things work with the US. The important battle being fought right now isn't specifically over Muqtada Sadr or some thugs in Fallujah, but rather it is over the hearts and minds of the moderate Shiite majority. If we lose the latter battle, the outcome of the former military one will not matter. I just hope Bremer and BushCo understand that.