According to the test machine at Pep Boys, the problem is the battery.

My guess is that both are bad at this point and the tester can't be sure.
So I replaced the battery (for the 4th time now IIRC), but this time with a regular Bosch battery instead of an expensive Optima or Orbital. The voltmeter is still reading ~11.6V at idle and ~13.0V at speed. Since I was short on time, I didn't have them re-test the car w/ the new battery... I guess I'll have to do that some other time. I bet the test comes back with "regulator bad" the next time around.