1) Iran is playing a very active supporting role;
2) Iran does not appear to be playing a unifying role. The Monitor story suggests that this is because it lacks the capacity to do so:
Although Iran is closest to the council and its affiliate parties like Badr and Sayed al-Shuhada, it's also backing many other Shiite groups in southern Iraq including those that are openly using violence to oppose British and coalition troops, according to Ali Ansari, an Iran specialist at London's Chatham House.
"The Iranians are backing as many horses as they can," he says. "But there is a limit to their influence, given how fractious Shiites are in Iraq."
There's an alternative interpretation -- it's possible that Iran lacks the interest. A fractious Iraq can serve as a buffer for Iran without triggering a security dilemma with Saudi Arabia or other Sunni states.
The Basra story is still developing, of course. Still, one wonders whether Tehran will be any more adept at nation-building in Iraq than the United States.
That final question is, I think, worth pondering. The common assumption is that Iran will back fill into Iraq in the wake of a US withdrawal, and essentially run the show, either directly or through proxies.(Your mileage may vary).
I'm not convinced that it's quite that simple or easy to predict. As the Guardian story Drezner cites indicates, Shia Iraq is a morass of competing factions, some of whom are supported by Iran, and others opposed to Iranian dominance in varying degrees. This is part of the problem in forming a national government. Of the three major blocs in the Iraqi government as we conceive them, Shia, Sunni, and Kurd, really only the Kurds are close to united. A recipe for political progress it is not.
Another important aspect Drezner doesn't address is that Iran's military/government structure is not simply a straightforward hierarchy from the Ayotallah Khaemeni on down. The Republican Guard, for example, is independent of the military, created as a sort of Praetorian guard in reverse to prevent a military coup in the early days of the Islamic Republic. It reportedly has it's own covert and intelligence elements. It is possible, even likely, that different factions within Iran's political and military structure are engaged in Iraq without close coordination. A wrinkle conveniently glossed over by war agitators and their stenographers in the national press.
To my mind, this muddy situation screams out for an effort to corral Iran into some sort of regional role as one of several to help restore a measure of political and economic stability to Iraq. An escalation of the current civil war among Shia could create a massive refugee crisis that Iran will have to deal with. A regional conflagration is perhaps not the golden opportunity for Iran made out by Neoconservatives and those trapped in the range of the beltway group think. The best leverage for that I would argue is a withdrawal deadline. Simply continuing to occupy Iraq only exacerbates the existing issues without exerting any pressure to change policy on the groups and regional players who are leveraging the current situation for their own purposes.
I'm not suggesting that Iran does not have a opportunity to benefit here, or that it has not been a bad actor, it has. I'm suggesting that like it or not, Iranian actions are the result of a complicated and sometimes contradictory political system, and that their capabilities and true interests might be more opaque than they appear at first glance.