I'm in the same boat as you...got a tv this weekend.
To get HD, Comcast told me i needed to either
a) get a cablecard
b) get a cable box
Either way i had to upgrade to digital cable. Then i looked at the plans on comcast.com. It seems you only get like 8-10 HD channels, 6-7 of which you can get free. The only non-free ones that i saw were comcast sports, espn hd (which i will miss) and discovery hd.
So i bought an over the air antenna. I'm not paying $17/month for 6 channels i should get for free + 3 extra channels, two of which i'll never watch.
I also noticed that i seem to have digital cable already, or at least a bunch of channels. During setup, my tv discovered like 80 digital channels (none of which look any better than analog, btw). There are a bunch of music channels and repeats like CBS, NBC, ABC. I guess they can't turn them off and depend on people not having digital tuners?
All of the non-HD channels look TERRIBLE. Even the digital channels that claim to be 480i or 480p. I guess i'm the oddball, most of the stuff i watch is on the broadcast networks.