To be honest, this is just me extrapolating based on all that I have read and heard. I do believe bandwidth is reserved on 750 Mhz or greater systems for several more national HD channels (at least in the Bay Area).
Here are some reasons we probably will see 2 to 3 more national HD channels on Comcast in the next 6 months (in no particular order):
1) HDNet/HDNet Movies: All the scuttlebutt is positive that Comcast and Mr. Cuban are making significant progress. Both play hardball in negotiations but both are rational and appreciate the respective advantages of HDNet on Comcast. HDNet is not just a satellite channel, with carriage on Time Warner and the other major cable companies (Comcast, Cox and Cablevision being the only exceptions).
2) Bravo HD+: NBC recently announced a "makeover" and more HD content for this channel. Also, the U.S. Open will be carried in HD there-- I know there are some big tennis fans among some of those Comcast execs in HQ.
More importantly, DirecTV's announcement of BravoHD+ carriage, combined with its existing carriage of HDNet/HDNet Movies, means that Comcast has to respond to this competitive development by adding one or more of Bravo, HDNet and TNT HD. Also, I suspect Dish has a deal for Bravo HD+ also, not yet announced, because it just negotiated a deal with NBC Universal for the HD Olympics and it would be surprising if Bravo HD+ did not enter into the negotiations.
3) TNT HD: Dish added this already, and its true HD content is increasing. NBA Basketball in HD on TNT HD will be a major marketing advantage for whoever carries it, even if it is only one or two games a week. Turner was scheduled to have completed its second HD mobile truck recently, giving it twice the ability to do sports/live events in HD. Also, Comcast and Time Warner seem to have a fairly close working relationship with a lot of joint ventures, so there should be relatively little difficulty negotiating a deal-- enough HD content is the critical factor.
In general, with DirecTV and Dish making significant HD additions lately (including NBC and FOX on DirecTV) and possibly more to come, Comcast will be behind the curve if it doesn't add at least one or two national HD channels in the next four months or so.
And, the big news lately on Wall Street is satellite taking even more customers away from cable.
Two words: Satellite competition. It is good for cable HDTV.
One final word: To the extent bandwidth is an issue in some areas, I think the next 6 months will show more bandwidth freed up by elimination of some analogs (or converting them to digital only).
And, the HDTV programmers (HDNet, Bravo, TNT) realize that Comcast does not have unlimited bandwidth and probably will not add all of them in the next six to nine months due to bandwidth considerations, so they will take that into consideration when negotiating. After all, they are competing with each other for carriage given the finite bandwidth resources.