Some full screen movies actually have all of the information that the widescreen version has, plus additional information at the top and bottom (not all full screen DVDs are pan 'n' scan). This is called Open Matte (the movie is shot in full screen, then mattes are used to block off the top and the bottom in order to make the movie widescreen). A good percentage of 1.85:1 movies (ones that fill or almost fill a 16x9 screen) are filmed this way. A good way to check is to view the DVD in the zoom mode that's designed for watching non-anamorphic widescreen movies (this mode should be a straight zoom, no stretch). If it looks like you're not missing anything while watching it, there's a good chance the movie is open matte. To create a widescreen version, simply crop the top and bottom of the picture.
For example, I have full screen versions of Doc Hollywood and the two Grumpy Old Men movies. When watching them, I simply zoom in on them like they're non-anamorphic widescreen DVDs. It's pretty obvious when watching them that they were filmed to be matted (the image doesn't look cropped). If I were to convert these for AppleTV, I'd use HB to crop them so that are are widescreen. The information I'd be cropping would be information that was intended to be matted, so I would actually be returning the films to their intended state.