I poked at it this morning and the downstream and upstream channels can get 1 channel synced fine. (Sometimes it looks like it can't because the modem keeps resetting the connection as part of its self-healing and so sometimes I look at the connection page and it's in the process of resetting.) So I think the cable line itself is fine.

When the channels are connected and synced, the modem's status page shows it getting through all of the connection stages except the last one: "Registration".

The Comcast-branded firmware that's loaded onto this modem isn't keeping logfiles and I can't find an option to turn that on. So I can't see the detailed reason why this modem suddenly stopped being able to register with the service provider. But at this point it looks like it's not actually a problem with the modem, it looks like maybe it's a problem with Comcast de-registering my modem's CMAC. Classy.

When I called them last night, they said they'd send out a technician (and then there was the problem that it's mid-week and I work a 9-5er and they don't have any pre-9 or after-5 slots this week) so I might have to wait til saturday for this to get fixed. However, I'm going to try to poke them on the phone and see if they can fix my CMAC.