Sadr has a militia, but no great role in the new Iraqi government. Now he wants such a role.
But like the man with only a hammer, everything looks like a nail to Sadr. I.E. he only knows to use his militia to further his aims.
It's even possible (though much less likely) that Iran approves of this sort of thing.
I doubt that this is a very successful process for Sadr, though. If he had the votes for Jafari, then he wouldn't have needed this attack...and without killing the local imam in this attack, he's unlikely to gain any new support, either.
As for "civil war" or other such possibilities, Iraq's militias and Mullahs are generally tied to a particular mosque. Thus, we know where they live.
This is an entirely different scenario than what we dealt with when Al Qaeda was itself the bigger threat in Iraq. We didn't know where to find Zarqawi. Not so for Sadr.
So we have different dynamics going on now.
Of course, the communist news media will focus on the violence and the negatives...neglecting to note the different dynamics (probably wouldn't grasp such distinctions even if they cared, anyway).
But dealing with a Mullah and Militia tied to a Mosque (a 3M problem) is a very different thing from that of dealing with a foreign financed terror organization that has no visible headquarters/leadership.
And perhaps the mullahs/militias are acting up *because* Al Qaeda has withered so much in Iraq. Al Qaeda's time has mostly passed now in Iraq. The Mullahs are going to give it a go next, but they have different goals than Al Qaeda.
This is a less dangerous game now. It is different. Beware the newscasters who tell you more of the same.
It's not the same now. The dynamics have changed.