Sorry for the long-winded post, but I finally got my unit working with Comcast, and I think this will help others:
First call was to the CableCard activation number: 877-405-2298. They got it "activated", but no encrypted channels. Some encrypted channels showed as authorized, but copy-once. Other encrypted channels were "not subscribed".
Next call was to the same number, and after several tries futzing with setting, she admitted that this is only an activation line and she'll connect me with "Cable Tech Support". They guy at Cable Tech Support tried for 20 minutes, tried to send a tech for a site visit (I refused), then gave up and said to call Tivo because he did "everything" and everything is OK on the Comcast end, so it must be the unit's problem. I think when I mentioned the copy protection flag, that was totally outside his realm of understanding.
I did try the activation web site: http://www.comcast.com/activate
, but it was down.
Third call was to the Comcast Tier 2 CableCard Support: 1-877-761-5015. Eric in CO (or maybe it was Derek as above?) asked for a ticket number. No ticket number, but I explained my history, and he tried a few things as I was poking around different channels. While we were chatting, he asked what it was. I felt more comfortable with him and said "a tuner". He asked if it was "one of those Homerun Primes". He said he's been activating a lot of those.
He seemed to have to do a few "resets", and it was strange because the encrypted channels started working, then after about a minute stopped working. He kept at it, and after about 10 more minutes, all the channels came back and stuck. Everything works fine in MythTV.