WaPo/ABC takes a poll:

Obama has proposed a timetable to withdraw most U.S. forces from Iraq within 16 months of his taking office. McCain has opposed a specific timetable and said events should dictate when troops are withdrawn. Which approach do you prefer - a timetable or no timetable?

Opinion on that question comes out 50-49 which goes to show mostly that it'll be deadly for progressives to let that kind of framing stand. The implication here is that McCain is hewing to some kind of agnostic middle ground about troop departures, letting the schedule be dictated by events. In fact, what McCain is hewing to is the goal of a permanent military presence in Iraq, and thus a military and political strategy in Iraq geared toward making a permanent presence possible. Given that such a presence is broadly unpopular in Iraq, and also a specific source of inter-factional tension and also a large incentive for Iran to play a destructive rather than constructive role in Iraq, it's a strategic objective that makes stability and substantial troop withdrawals essentially impossible for the foreseeable future.

The idea of a 16 month timetable sounds a bit arbitrary because it is a bit arbitrary -- why not 15 months or 17 months? But a certain level of arbitrariness is inherent in the idea of setting a fixed schedule. And a fixed schedule for withdrawal is the only context in which it's possible for US forces to accomplish something constructive during the remaining time, will let us reallocate resources away from this wasteful war, and with some luck will actually reduce the level of internal tensions in Iraq. There's no choice between setting a timetable and taking a "wait and see" attitude, there's a choice between putting down a marker (in the real world, more likely negotiated with the Iraqi government than inside a presidential campaign staff) of where the exits lie, and a costly and pointless open-ended engagement.