The drop off in violence comes because the Shia have been successful in purging mixed neighborhoods of Sunnis and encroaching on Sunnis neighborhoods. My money is on this unpleasant scenario. We are likely to see in the coming months a consolidation of Shia power in Baghdad and, for all practical purposes, it will become a Shia city.
It is on this point that the press must stand up and do its job. If the violence falls off we must know if it is a genuine result of accomodation and truce or, more likely, the result of consolidation of Shia power. If it is the latter then we will know that General Petraeus has failed and that the time to withdraw U.S. troops from Iraq has come.
Of course, given how desperate the remaining few supporters of the war have been to spin changes as a positive sign, there is little likelihood of such truths being revealed.