No, that is a problem with satellite, not cable.
If you get Comcast, every TV in your home with a cable outlet will be able to receiver standard programming (no extra cost) your plasma will need not just a DIGITAL cable box, but the cable box must be the HDTV digital cable box.
There are two (actually dozens) different models of digital cable boxes. The normal type just receives the non-HDTV digital channels (about 400) and the HD digital cable box also receives the HD channels along with the digital channels. Both boxes also get the analog channels.
So, yes, with a HD digital cable box from Comcast you will get your locals, HD channels, and digital channels with nothing else necessary. Check your local Comcast website to find out your exact channel lineup.