If the film was shot in the scope ratio (2.35 - 1) it will still be slightly letterboxed even on a widescreen TV as the image is still wider than the TV's.
A film shot in the, now pretty standard, 1.85 - 1 ratio translates to the full 16:9 image of a widescreen TV without losing anything of note.
An older movie, in the academy ration 1.33.1 will sit in the middle of your screen with black borders at each side.
Avatar is an exception to this. It was shot 1.85.1 (hence the full screen dvd) and masked off to 2.35.1 for the cinema. You're actually gaining picture on the dvd and bluray.