Welcome to the forum, Blindpilot!
The HDTV upgrade with Comcast will come with HD signals for broadcast channels (PBS, CBS, ABC, NBC, CW, FOX, etc.), and a small smattering of cable channels like Discovery, National Geographic, TNT, HGTV, Food Network, A&E, and ESPN networks. It also includes some "minor" HD channels like MOJO and MHD - concerts and such in high def.
There are a few HD premium channels like Showtime, Starz, HBO, Cinemax, etc. that require further subscription. Otherwise, all of the above comes through when you hook up the HD box.
As an aside, the box my family received has DVI and optical audio outputs... no HDMI. I'm not sure if all Comcast HD boxes are created equal, but I thought I'd throw that note in so you can be prepared with the right cords...
