Why not just change the MAC address? Point you browser at the cable modem's IP address and I'm betting there will be a configuration page there, and that the MAC address will be one of the things you can configure.
1) I didn't think of that, because the MAC address is printed next to the serial number. 2) As it turns out, you can't do that anyway, but I appreciate the suggestion. I wish I had thought of that last night. 3) Much as I enjoy hating on Amazon, they did refund me the money, 11 months later. So I have a new-in-box one showing up tomorrow, which should be soon enough.
If you're not using a router + modem combo, you can change the MAC on the router to whatever Comcast expects it to be. The modem can be set to act as a bridge and just forward along packets from the router. I discovered this in August after plugging my laptop directly into the modem. Two routers later, it's still set to lappy's MAC address.
I shoulda asked you guys first. Unfortunately the only viable computers at the moment are Macbook Airs, which have no ethernet ports (and the modem isn't a router, nor is it wireless). Fortunately, Amazon is sending me a new one. Comcast doesn't give a rat's ass about the router anyway, it's the modem they hate, and all sorts of internet warfare tells me that the SB6141 has an unalterable MAC address.
Stupid thing is I've got a FIOS box on the house. Unfortunately Verizon sold their operations here to Frontier, which will charge you $59/mo for 30mbps and won't refund your money when your internet stone cold stops for hours at a time. Comcast's hinkyness notwithstanding, at least it's on.