I've had Comcast's HD offering since 2/2002. They provided a sidecar hookup (HDD-200) to my digital STB (DCT2000). Once connected, I was able to get HD on NBC(Phl), ABC(Phl) and HBO. They offer Showtime as well, but I don't subscribe to it.
I've been questioning them weekly about additional offerings such as Discovery, CBS, etc. , but can get no firm answer on a schedule. Price for rental of the HDD-200 is $12/mo.