The local cable company, Comcast@Home, rents you the cable modem as part of the $49.95/month. That includes $7/month for the modem rental. If you have your own modem (you can buy their Toshiba modem for $300), then you don't pay the $7. So for $100 of other stuff, this is GREAT! The savings would pay for the modem in just over a year (assuming this deal happens) plus you get the $100 in stuff!
MAC addresses are built into the hardware in all modern devices so you cant change em. If you get your own modem, you just have to supply @home the MAC address and they update their switch to allow the new MAC address through.
I have DSL, so I'm not really in a hurry to change. Better connections, just lower bandwidth. (Jebus, I hope this thread doesn't deteriorate into a cable vs DSL debate.)