Different HDTV signals?

Yes The above is correct, when the program is broadcast in standard definition, you will see bars all around your screen, when they are broadcasting in High def, you will view it in widescreen. Perfectly normal. If you are not getting any widescreen broadcasts during primetime your cable company is not transmiting them in the High Def format. If some of the time on the networks you are getting widescreen , thats because some shows like NYPD Blue are produced in high def, while a show like Friends is not.
Also many cable companies with high Def capability broadcast the networks in High def on different channels on your cable box , For example CBS is channel 2 in NY/NJ but if I want to view it in high def on my system it is channel 183.

real high definition programming is in wide screen 16:9 aspect ratio anything other isn't really actually high definition its being converted from something else to sort of high definition.

All of the above posts are correct. I just want to add some minor details. For HD channels only, such as channel 904 for HD-NBC rather regular channel 4 for NTSC NBC in my case, prime-time shows broadcast in HD would fill up the plasma screen. When it comes to commercials, those are not in HD. Therefore, they don't fill up the entire screen, and the black bars show up, so whenever the signals are not in HD, such as commercials or non-HD programs, the TV screen won't be fully filled.

Oh, lets add to that list; the many movies on the pay channels that aren't in oar. Some just weren't shot in widescree and some they just don't use the widescreen version / transfer.

I recently got a plasma display, and the aspect thing surely is a pain. My remote has a little red button to change aspect, and I expect it will wear out long before the plasma dies.