I think it all depends on the combination of your monitor resolution, playing back the AHFilm shot fullscreen, and the screen size that FRAPS is recording. Add to that the file aspect/size you set your project to in the movie editor when you import your recorded AH shots. I doubt it has anything to do with how you set up your shots in the film viewer (world relative versus plane relative).
Try setting your screen resolution to 720p specs, play back AHFilm in full screen, and record FRAPS as full screen, as long as your project is set for 720p in Sony Movie Studio, it should import without letterboxing.