My new Motorola cable modem arrive last week and I hooked it up. Unfortunately it reports "IPv4 only" - I guess Comcast hasn't enabled IPv6 in this area. So I'm stuck using the 6to4 tunnel.
Device specifics: SURFboard SB6121. Hooking it up was painless. I called 1-800-COMCAST. The tech first asked for the device serial number - I've heard they do a stolen device search because people take the equipment with them when they move and sell it on eBay. Next he asked for the MAC and put me on hold for 5-8 minutes. When he came back on the line it all worked.
Word of warning, if you are using a VoIP phone, your old modem will be disabled during the call. Consider calling from a cellphone.