I flew American from the US to the Virgin Islands to get married, and the Flight Attendants put my dress in their little closet. This was pre- 9/11 and pre-insane carry on restrictions, though.
I think the majors (BA, Continental, United, etc) would most likely be accommodating in terms of hanging your dress, etc. Be very nice to the ticket and gate agents, and ask them if there's any way they can help you keep your dress from getting wrinkled and they may be able to talk to the flight attendants for you.