Maybe a dumb question, but obviously for DVD/HD DVD you calibrate with the appropriate disc. Without a disc to calibrate one's cable input, what is the next best thing? Would you calibrate your cable input using a 480i signal from your DVD player?

Without a disc to calibrate one's cable input, what is the next best thing?Depends on your connection. If both your cable STB and DVD player are DVI/HDMI and you setup your DVD player for 'video' levels (16-235), then the STB will give the same results on the same input.

Otherwise use the HD-NET test patterns (http://www.smartcalibration.com/hdnetpatterns.html) which are broadcast at some ungodly hour in the morning for 10 minutes on weekends (check the HD-NET listings).

Maybe, assuming the sources are outputting exactly the same thing. Which if analog is probably unlikely.

In any case, broadcast and cable is so all over the map channel to channel, it's probably best to use a baseline like a DVD player or signal generator and then tweak by eye from there. I hate to recommend tweaking by eye, but there isn't a reliable reference with TV unfortunately.