I work for adelphia (soon to be comcast).  Every channel will be digital or high-definition within a few years, per order of the F.C.C..  But if the channel itself doesn't broadcast their shows in H/D, the cable company can't send it out that way.  That's why if you notice, channels 4,5,6, etc. only broadcast their shows in H/D after 8:00 or so, its because it costs more money to send it out that way.  So they only choose to show them in H/D during primetime hours.  Hopefully that helped.
