The combo PVR/satellite boxes don't work with cable. Your options are Tivo standalone, or ReplayTV. Both have IR emitters that control channel changing on the cable box. With certain models of cable box, on Tivo at least, you can get serial control to work, somewhat more reliable.
Scientific Atlanta does make a combo PVR/cable STB, but it's only currently available in some areas served by Time Warner. I think all the major STB makers have PVR models available/in development, but you have to wait for your local cable company to decide to offer them ...