Al-Sadr's militia is called the Mahdi Army. Yes, they are Sadrists who speak the same language and share the same religious beliefs of those in Sadr City.

Why do you think that an army of occupation (the U.S.) can do a better job than the Mahdi in Sadr City?

The Kurds in the north have been relatively quiet because they have not been in the direct line of fire between the Sunnis and Shia. The Kurds were hoping to stay out of the civil war and eventually separate from Iraq. Now that the surge is underway in Baghdad, the insurgents have started targetting the Kurds.

The U.S. is running around trying to put out fires but there is nothing they can do to prevent the Shia majority from ruling Iraq. Saddam was able to do that but only because he was a tyrant. The U.S. has given the Shia the opportunity they have been waiting for. Not at all what Bush was hoping for but heh - you win some, you lose some.