Different HDTV signals?

All HDTV is 16:9 widescreen. It sounds like you're getting a non HDTV digital feed from the cable. Also, remember that not all network shows are broadcast HDTV.

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.