I just got my Sceptre 42 inch 1080P LCD from costco, and it came with a ATSC+QAM tuner. Despite my digital cable box having HBO/Starz/Encore and about 200 other channels, the ATSC/QAM tuner on the TV can only get 7 HD Channels however, and about 42 total channels (mostly garbage). The high def ones from comcast are ABC, NBC, Fox, CBS, PBS, UPN and WB (yeah who knew UPN and WB had high def channels).
The reason Im not getting HD ESPN, HBO, TNT and the INHD channels is that comcast is only required to broadcast the over the air channels unencrypted to their subscribers. All the other channels require a cable box that can handle encrypted QAM signals. At the moment the only way to accomplish this is via a Comcast set top box, or with a comcast cable-card solution inside of a TV.
At the moment,the only way you can capture all the HD and Digital channels from comcast and most cable companies is if you have a VIVO card which can take Component input from your cable box.