The answer to this question is generally a yes.
Many sponsors of photo contests and competitions are unashamedly robbing photographers of their work and stealing their creativity. Photographers submit photographs to a contest, the terms of which take away their rights to their submission; hence providing the contest's sponsor a 'free' photo which can then be sold or used at will for advertising purposes.
Pro-Imaging has a run-down on this issue, and lists a number of photo contests for you to watch out for. Generally-speaking, the majority of photo contests are rip-offs in one shape or another. Yes, some may argue that having a winning entry is good for their careers and widens their exposure, and I acknowledge that this has some validity...but that's exactly what sponsors of such contests prey upon. Seeing one's photograph winning a National Geographic or Smithsonian contest is enormously thrilling, but seeing it being subsequently used for other purposes will eventually make you realize that your work is being exploited.
So read the fine print carefully, and make an informed decision.
Pro-Imaging's Rights Off Lists