Voom has a lot more HD channels, but most of those channels are upconverted programming, not true HD material. Most of the programming is the same stuff that's broadcasting on the SD channel, just upconverted to HD.
ESPN HD, HBO HD, Discovery HD, Showtime HD, HD Net, HD Movies and PPV HD is what's currently available at this moment. I read a post the DTV will be adding 11 more by August or September, but that's just talk right now.
To get your local digital channels you will need an off-air antenna. The FCC has not given the Satellite companies permission to transmit digital channels via the satellite.
CBS-HD is only for a select few people who qualify.
To get HD channels you need a compatible receiver. Also, a subscription to locals and the HD packages. Locals channels can be digital, but not be an HD. Only certain programs are broadcast in HD.