Here in Cary, NC, the local cable company (Time Warner Cable) offers HD broadcasts (HBO, Showtime and all of the local affiliates currently doing HD) over their digital cable network. Of course, you do need an HDTV to view HD images and you do need an HD-capable digital cable tuner to decode the cable signal into HD. However, they charge no more for the HD-capable digital cable tuner than they do for the "regular" digital cable tuner. They also do not charge extra for the HD channels. Since those channels have to be broadcast within the digital tier of service, you do have to subscribe to the digital level of service. If you are paying for the HBO collection of channels, you also get HBO in HD at no extra charge. Ditto for Showtime. Of course, you do have to tune into the correct channel to get the HD version of any of the supported channels. If you get CBS on channel 6, for example, tuning to channel 6 will NOT give you HD images. You have to check the channel guide and find out which channel they use for the HD version of CBS. It will be within the digital tier of channels, so it could be 101 or 252 or whatever.

Now, some local stations actually have multiple digital channels and may broadcast in a combination of resolutions. My local CBS station runs an analog station, an HD digital station (mostly the same shows but some HD documentaries instead of game shows or whatever at different times of day) plus a standard defintion digital station that is a 24-hour news station. During the March Madness basketball tournament, they changed over to four standard definition digital channels, so you could watch any or every game. Since they also own the local Fox station, they broadcast any game that was being shot in HD over the Fox station in HD. It was a basketball lover's wet dream.

Of course, that's just the way they do it here. The cable company in your community may handle it entirely differently (charging extra for the HD tuners or the content or whatever). Some restrictions apply. Your mileage may vary.