I would say that the decrease in violence in Iraq in 2007 is not solely the result of the additional five brigades, nor is it fully attributable to the success of counterinsurgency theory (although, I would argue that they did, in fact, play a key role). Certainly the Sunni Awakening--well underway in late 2006--played a significant role in the decrease in violence, as was the stand-down of the Mahdi Army, a Shia milita. Additionally, some studies have also noted that, by early 2007, many of the mixed neighborhoods, particularly in Baghdad, had completed the process of driving out or killing their respective minority groups, leaving behind homogeneous and far less violent areas.
The decrease in violence was not the result of any one factor--rather, all of these elements came together at exactly the right time to quell the violence in Iraq...at least for right now. Let's see how tenuous the peace is when American troops begin to withdraw next year.