The most important improvements in Iraq have little to do with the US troop surge and even less to do with the central government.
Although it's true that backing Sunni groups is reaping big gains, the success of that strategy has little to do with the surge and, more importantly, if poorly managed may unleash forces that undermine the ultimate goal in Iraq.
Any opportunity for improved security in Iraq should of course be seized. Engaging Sunnis provides one possible bridge to a substantial withdrawal of US forces. But this strategy also comes with risks.
Sunni groups also want to reverse their current marginalization and position themselves vis-à-vis their Shiite counterparts, and Iran, in the event of a US withdrawal. It is enemy-of-my-enemy logic, not a change of heart or US troop increases, that is driving Sunni cooperation.