Another visit from a Comcast tech, and the CableCARD still won't activate.
Worked with SiliconDust tech support. They accessed the Prime remotely to check the logs, and found that it has a hardware issue. I was issued a RMA with a UPS shipping label. Meaning, they're footing the bill for shipping both ways. Nice!
I'm pretty impressed. Hopefully the new one will work and all will be smooth sailing. =D