Jason J's link is an excellent resource. Basically, though, it's a simple math problem: widescreen HDTV's are an aspect ratio shape of 16:9 (or 1.78:1). Movies are shot at typically either 1.85:1 or 2.35:1, meaning that both are wider than HDTV displays. The result is that both still need at least a little bit of letterboxing to preserve the original aspect ratio.