The Command Post has a post up saying:

“The U.S. troops arrested radical Shiite Imam Muktada al-Sadr in Baghdad on Tuesday. The report came from Hazem al-Araj, his closest associate, who is the head of the Baghdad bureau of the ?Mahdi army? group.
Looking to either confirm or deny. More as we find it”

I’ve not seen anything other than this, but I’m watching.