Mine does the same. I get fullscreen (16:9) for the SD promos and 4:3 for the HD episodes.
I think I can explain this. The SD promos are stored on the disc in 480p (720x480). This is a 4:3 resolution. Your TV detects this and stretches/zooms/whatever to 16:9.
For the episodes themselves, the files are 1080p, and the resolution is 1920x1080. This is 16:9. The left and right of the image have black bars. (See here: http://tng.trekcore.com/bluray/exclu...Farpoint15.jpg
) Most TV's will see the content as being 16:9 (which it is, technically) and not alter it. The black bars on the left and right are stored as part of the video image itself.
Hope that makes sense.
That makes perfect sense. Thanks!