Apparently it was US intel that convinced al-Maliki to target al-Sadr:

Iraq’s prime minister has dropped his protection of an anti-American cleric’s Shiite militia after U.S. intelligence convinced him the group was infiltrated by death squads, two officials said Sunday.
In a desperate bid to fend off an all-out American offensive, the radical cleric Muqtada al-Sadr last Friday ordered the 30 lawmakers and six Cabinet ministers under his control to end their nearly two-month boycott of the government. They were back at their jobs Sunday.
Al-Sadr had already ordered his militia fighters not to display their weapons. They have not, however, ceded control of the formerly mixed neighborhoods they have captured, killing Sunnis or forcing them to abandon their homes and businesses.
[…]
Prime Minister Nouri al-Maliki’s turnaround on the Mahdi Army was puzzling because as late as Oct. 31, he had intervened to end a U.S. blockade of Sadr City, the northeast Shiite enclave in Baghdad that is headquarters to the militia. It is held responsible for much of the sectarian bloodshed that has turned the capital into a battle zone over the past year.
[…]
“He told the sheik that the activities of both the Sadrist politicians and the militia have inflamed hatred among neighboring Sunni Arab states that have been complaining bitterly to the Americans,” the official said.
Sunni Muslims are the majority sect in key Arab countries like Saudi Arabia, Jordan and Egypt, all of which have shunned al-Maliki. Shiites, long oppressed by Iraqi’s Sunni minority, and vaulted to power with the ouster of
Saddam Hussein.

Everyone is blathering on that the troop buildup won’t be effective and will fail as it has done in the past but if these reports turn out to be true, then it will be a lot different because now the military will have the support of the leadership in Iraq to go after everyone, not just the Sunni.
Related posts:
The New Strategy in Iraq
Sadr is no longer safe
General Petraeus’ Strategy in Iraq

400 Shiite detained in Iraq
Top al-Sadr aid captured
Updated: Allah is skeptical. I have no idea, I’m just a SAHM and don’t know anything about war.