Well I can't speak for Comcast, but on my Plasma, almost any non-HD channel looks crappy. But, thats the nature-of-the-beast with Plasma's. So yes, on plasma's regular channels look worst than on TV's or LCD's.
But, I still wouldn't trade my Plasma for ANYTHNG! You should see this thing running an DVD, through my HD DVD player & my sounds system. The new Stars Wars trilogy rocks! I just have to keep the sound down low enough, so I don't smash the windows.
Dave...