What TJM said. I was about to write a blog post to that effect (really). I especially agree with Th's argument in support of TJM.
All I have left is the chance to note one odd thing. There is a bit of symmetry between the surge and the deadline -- both make combatents lie back and wait for it to be over.
please read on before biting my head off.
It appears that the surge has caused a shift of violence from Baghdad to Diyala as fighters are avoiding direct contact with the surging troops (esp the Mahdi army guys). Thus the "promise" consists of convincing Mahdi army fighters to lay low. On the other hand a deadline would be terrible because they and the sunni insurgents would lay low all over Iraq.
Now I think I can oppose the surge and support a deadline without falling into contradiction. For one thing the surge has direct costs to our overstrained military and the deadline does not. For another, the flypaper theory is absurd while there is solid evidence that the Baghdad surge has lead to increased violence in Dayala (the Mahdi army and the insurgents are not going to leave Iraq while laying low). Both Moqtada al Sadra and al Douri (close to insurgents) have said their condition for a cease fire is just that deadline. Iraqis are not convinced we don't plan to stay forever. A deadline for withdrawal would lead to actual withdrawal which I support (as do most Americans).
However, I don't see how it is possible to support the surge, which is explicitely of finite duration, while opposing a deadline for withdrawal. Both lead to fighters laying low. Is that a good thing or a bad thing Mr Brownstein ?