GF just did the exact same thing (Earthlink DSL->Comcast Cable) herself.

You do not need your computer to be directly connected to anything. The cable modem is like any other internet gateway. Her connection is like this:

Wall plug->
Cable Modem->
Wireless router.
Then she connects all ethernet gizmos wirelessly.

She does have VOIP telephone service as well, and the VOIP decoder box has to be plugged into the wired portion of the network somewhere near the cable modem, but I don't think that's what you were asking about.