The entire Sadr rebellion was an attempt to preempt Sistani, and more importantly, Sistani's people, from getting ahold of the State machinery by means of a mosque election. Sistani will put the word out through his mullahs to vote the straight Sistani ticket. The Iranians know this, and want to prevent Sistani from setting up a "rival school" to the Qum Ayatollahs. Sistani, operating out of Najaf and Karbala, has a legitimacy that the Teheran crowd does not have within the Shi'a cosmology. All Sistani needs to do is to make a deal with the Sunni collegium. A deal with the Kurds and their powerful Peshmergha is pretty much done. With Sadr out of the way and the Marines on their way to rubbing out the Sunni rebels in Fallujah, most of Sistani's problems have been solved for him.
The Coalition gets to do the unpopular heavy lifting, leaving Sistani the opening he needs to pick up the pieces. The Iranian covert actions were clumsy and ill thought out. They should have approached Sistani with a view towards making a deal, rather than promote their own toady, Sadr. Once Sistani is victorious, he will owe Teheran nothing.
All Sadr had going for him was his family name. Now he has nothing, and in the end, so will the Iranians.
Be Seeing You,
Chris