Read an interesting tidbit in the current Aviation Week and Space Technology issue. It seems that Air Tran doesn't have a yield management system in place yet (can't afford it) so they overbook all of their flights by 5 pax (for full flights, that is). This might be of use to you if you want to get bumped, it is a guessing game with the other airlines because their yield management system dictates that some flights are way overbooked and others aren't overbooked at all. Supposedly this results in fewer bumps.