I get my signal from Time-Warner Cable, though I could get our five local stations over the air for CBS, NBC, ABC, FOX and PBS.
One of the odd things that happens is that on cable the stations get a chunk of bandwidth and will split it up into a couple of standard def versions with different content., often to the detriment of the high def signal. For example WRAL here has an all weather and all news feed, but there HD signal has such crummy compression you lose a lot. My understanding is that over the air is the same way.
On the other hand, some of the cable fed HD, like DiscoverHD, PBS, ESPNHD and others are literally stunning.
If you want a crash course in HDTV, home theater, hardware, etc you need to visit the FR of HighDef, the avsforum.com.
Yup. That, and the multiplicity of HD standards will be an ongoing battle of years to come.