http://www.airsafe.com/complain/bumping.htm
U.S. airlines are allowed to overbook flights to allow for "no-show" passengers. However, if passengers are involuntarily bumped, airlines are required to do ask for volunteers to give up their seats in exchange for compensation. Most involuntarily bumped passengers are subject to the following minimum compensation schedule:
* There is no compensation if alternative transportation gets the passenger to the destination within one hour of the original scheduled arrival.
* The equivalent of the passenger's one way fare up to a maximum of $400 for substitute domestic flights that arrive between one and two hours after the original scheduled arrival time or for substitute international flights that arrive between one and four hours after the original scheduled arrival time.
* If the substitute transportation is scheduled to get you to your destination more than two hours later (four hours internationally), or if the airline does not make any substitute travel arrangements for you, the compensation doubles to a maximum of $800.