There is actually a bit of a quandry out there in that you are supposed to calibrate a system, not just an input. So, with a calibration DVD you are essentially calibrating your DVD path (plus audio which is not dependent upon TV input, etc.)
Unfortunately, the best way to calibrate a cable or antenna HDTV system woudl be to have a dedicated channel that is running ISF content. That's not going to happen anytime soon as far as I can tell, so the next best thing is to use he DVD to calibrate the input for that device.
Remember, on most TVs/displays each input retains its own calibration settings. In other words, if you calibrate your component input for DVD, but use s-video for cable - you have done nothing for the cable input. The next best thing in this case is to hook up the calibration DVD to your s-video input and calibrate that way.
Make sense?