It seems many recent news reports on Iraq, TV especially, have failed to stress that much of the fighting with Shi'ite militias is taking place in areas technically outside the U.S. zone of responsibility. Already some allied forces have pulled back, while others are asking for U.S. help.
This is why U.S. commanders wonder if they have enough troops on the ground. The implicit deal with Coalition partners, with the exception of the Brits and possibly the Poles, is that they do some peacekeeping and beat back the occasional lawless outburst, but as for sustained offensive operations, no thanks.
Of course, offensive ops is exactly what the U.S. now plans. You do not have to agree that "an Iraqi version of the siege of Waco" is likely in Najaf to see that American forces will face some dicey calls in the near future.
Meanwhile, Moqtada al-Sadr obviously sees some potential for scaring off some Coalition forces.