I always guessed that using a cable connection with and ADSL router would work, but using an ADSL connection with a cable router wouldn't. For the reasons VHG mentioned.
ADSL routers tend to be more expensive because they have to be able to communicate with a phoneline, whereas cable routers have that done for them. Surely if the ADSL router has an ethernet input then it would work? Since the output of a cable modem is basic PC network speak, couldn't you just attach it to the ADSL router as if you were using it, as, well, a router?