From his appearance on 60 Minutes last night:

"At a time when American casualties are down, at a time when the violence is down, particularly affecting the Iraqi population, is that the right time to try and set time tables for withdrawing all American troops? I mean you talked about…the end of 2009," Kroft remarked.

"Yeah, absolutely. I think now is precisely the time. I think that it is very important for us to send a clear signal to the Iraqis that we are not gonna be here permanently. We're not gonna set up permanent bases. That they are going to have to resolve their differences and get their country functioning," Obama said.

"And you pull out according to that time table, regardless of the situation? Even if there’s serious sectarian violence?" Kroft asked.

"No, I always reserve as commander in chief, the right to assess the situation," Obama replied.

So the withdrawal of U.S. forces is contingent on the security situation in Iraq? This is not much different from McCain's position, or Bush's. The disagreement then is over timetables and permanent basing, but the timetables will be disregarded if they are ineffective, and the basing is a foregone conclusion--the Iraqis will be dependent on U.S. airpower and armor to defend their borders for the next decade at least. His position is evolving, and if he becomes the nominee, we're likely to see it evolve even further.