Are you using a cable/sat box? In some cases where symptoms like this have been reported, the STB isn't set to output in the 16x9 aspect ratio. Try adjusting (checking) that to make sure that it's set to output 16x9.
I just bought the 50 inch Samsung Plasma PN50C550 and have a problem that I cannot seem to fix. Every time I change channel that has a different resolution (1920x1080i, 1280x720,720x480), the TV will not adjust the picture size accordingly and may be missing part of the image either on the bottom or the right and side with a black bar only on the left. The only way to force it to correct itself is to do a picture Self-Diagnostic. That will only last as long as the resolution of another channel remains the same, otherwise I have to constantly do the test. Besides that, the TV has a great image quality, but most likely will return it if unable to fix this somewhat annoying issue. Any help would be appreciated.