Comcast in my area only logs the modem I think. When I switched to the 825, I don't remember doing anyhting with its mac address.
Must be a regional thing.
When I first signed with Comcast, they wanted the MAC addresses of both my router and modem. Later when I switched out my old router for the DIR-655, I couldn't get a WAN connection. Then BINGO I was back up and running immediately after I cloned my old router's MAC.