Originally Posted by Doppy
This happens to people who board at the end all the time. If it doesn't fit underneath the seat in front of you and they don't have anywhere else to put it they'll give you the option of gate checking it or getting off the flight.
The airline's own brochures say not to put valuables in checked baggage. If the airline insists on taking your properly sized suitcase and you have valuables in it, you may get off and should insist that the airline still use its best efforts to transport you without collecting any additional fees or fare.
Travel tips:
http://members.aol.com/ajaynejr/travel.htm