While all DirectTV channels are digital, only a limited number of the channels carry HD programming. Current HDTV channels include the HBO and Showtime channels, Discovery-HD, ESPN-HD, and HDNET.
These HD channels are transmitted as separate channels in the 70 to 90 range; the same programming can typically be watched on their corresponding standard channels. HDTV programming uses more bandwidth than the standard DirectTV channels and are only available bundled as part of premium packages with up-charges of $10 to $15 per month. While you can receive standard definition local channels from DirectTV for $6 per month, no local HDTV channels are available.
To receive HD programming through DirectTV you need a three lnb oval dish and a satellite receiver capable of decoding the HDTV signals. This is NOT the equipment you are provided as part of the DirectTV teaser offers which promise free hardware.

DirectTV’s HDTV info: http://www.directv.com/DTVAPP/imagine/HDTV.jsp