It appears to be a regional thing:
Comcast to Add 30 HD Channels In Seattle
http://www.tvpredictions.com/comcast070209.htm
If you are on your Comcasts emailing list - they will send you an email when they are going to add HD channels.
I just got ION HD (which has almost no HD to speak of) which is a local station (Miami) that is out of my area (Ft. Lauderdale)
Lots of promises but only results count.
That link I gave you - that website is very good about cable companies adding HD channels.