Iraqis fire surface-to-air missile at chopper over Sadr City - On Deadline - USATODAY.com

The cease fire in Sadr City appears to be holding for the moment in Baghdad, though if I were a US military man, I would be rather concerned at the report that someone, probably a Mahdi Army member, fired a surface-to-air missile at at a helicopter before things settled down.

This was likely not a lone combatant making this decision. This was likely a conscious decision made as a warning to the US that the militias have resources that could mean that air operations over the Baghdad slum will no longer be quite so easy.

For some context, I would suggest that this article from Think Progress goes a long way towards explaining Muqtada al-Sadr's popularity. Money quote:

One of the central elements of the elder Sadr’s program (and now of Muqtada’s) was a distinction between the “silent clerics” (represented by Sistani and the Najaf establishment) — bookish sorts who stay remote from the lives of their people — and the “speaking clerics” who take part in the suffering and struggle of the Shia, as Sadeq did. And here the “silent clerics” once again stayed silent while Shia were crushed in Sadr City, of all places, while medical care, food, and shelter are being doled out in Muqtada’s name. It doesn’t require any math to see that Sadr benefits politically from this.