My local Fox station just started broadcasting in HD and they just added it to the Comcast QAM-256 feeds.
I had the other local HD channels setup previously, but when I tried to update my myth backend with this new channel I can't seem to get it added. I know it's on channel 60 because I verified it on my cable box in the diagnostic mode. The problem seems to be the mythtv-setup. When I scan for the channels it gives a "no tables found" error for that channel. I tried deleting all channels for that tuner and did a full scan with no luck. I checked the database and see that the channel is there but I think it's missing the PID that tells the tuner what stream to use. 
update: After playing around with the QAM feeds on another machine I was able to get it to work. I think the no tables found error is common since comcast strips out all the program ids. On the plus side I also discovered that they are also simulcasting the OTA channels in SD as well as HD.
That will come in handy for my SD frontend.