With fuel prices and airline tickets where they are, US is the least likely to go:
On TATL flights, they offer the most competitive rates and I can testify about their excellent service.
US has announced the smallest cuts of all U.S. airlines and will definitely be the last to go down with CO, Delta.
I think that United and Northwest are the most likely to go and I see NW going down before the merger with Delta completes, around December maybe.
AA is a mistery to me, can't really judge.