Hopefully this will help someone else who runs into the same thing. After I'd tracked down and installed several older drivers (last being dated 8/15/06), none of them working, I tried something on a whim.
I shut down the ehshell and the scheduling services (both of the running Media Center services), then opened Device Manager.
I then brought up the properties for tuner 1 and clicked on Update Driver, then let it pick the best one. It installed one from January '07, must be latest. Okayed that and repeated for tuner 2.
I restarted the two Media Center services, then started Media Center.
Now when I tried to Set Up TV, it worked! I went through and configured for DirecTV (FIOS TV gets installed tomorrow) and it all worked. I didn't see an option for a FIOS TV guide, but I wonder if it shows up if you pick Cable instead of Satellite. Hey, it's a fiber optic cable, right? ;)