The general consensus is that the militia has been seriously weakened in the past year by Iraqi and U.S. forces bent on taking them out of the equation. The organization already announced to reorganize back in June to a more political organization.
What would any self-respecting terrorist organization would do when faced with annihilation? Well, join the political process of course. It's a slick move by the militia. By linking disarmament to a U.S. withdrawal, they place pressure on their own government to get a deal done. Whether or not this move by Sadr's militia impact our own stance on negotiations remains to be seen. In the effects business, we call this good IO (Information Operations).