It's the same method they use for phone-over-cable; they just unhook your external phone connection from the
LEC and connect it to the modem, which then plugs either into the cable network (in my case) or the FIOS network.
Don't bother fishing cat-5; go for gigabit wireless (802.11n). Much less hassle.
ETA: sorry I didn't see your post in time! I certainly could have answered it.