I am a Comcast HD subscriber in the Philadelphia area, using the Motorola DCT-5100 STB to feed HD to my Sony GW-II.
You almost certainly need a STB from Comcast (most likely the Motorola DCT-5100, although I believe in some areas Comcast still uses a Scientific Atlanta HD STB). As for available network HD programming, it depends on your particular area (a Comcast CSR should be able to at least correctly tell you what is currently available in your area in HD). In Philly, we currently get ABC, NBC PBS and Comcast Sportsnet (a Philadelphia region all sports network that does Flyers/Sixers/Phillies). Comcast does not deliver CBS in ANY market, such as Philly, where the CBS affiliate is owned and operated by CBS (Viacom). Without casting aspersions, it appears (as reported in several other threads) that Viacom wants some fee for the CBS-HD feed, and Comcast is unwilling to pay. My understanding is Comcast does deliver CBS-HD in some, but not necessarily all, markets where the CBS affiliate is independently owned (in those cases, Comcast may have negotiated a carriage agreement with the local CBS affiliate). In Philly, as is most likely the case anywhere for Comcast, HBO-HD and Showtime-HD are available if you subscribe to HBO or Showtime, respectively.
I am not holding my breath on CBS or any other new HD programming, particularly ESPN-HD, based on what I've read about the situation (that's not necessarily any different than the situation with D*). As noted in a recent Forbes article (look for the thread), Comcast is apparently trying to reduce its programming costs and is using its significant market share to take a hard line in negotiations with content providers such as Disney and Viacom. There were reports months ago about negotiations with HD-Net, but I have not seen any recent updates.