As a follow-up... I got my phone unlocked today. I decided I wanted to test out AT&T over the weekend, so I stopped into Radio Shack to pick up a GoPhone SIM for $10. It's apparently something new for them to sell, as the guy didn't actually know they had them. I had to tell him that the website said they had them in stock in the store, and then he found them in back. (Tip for anyone else who does this: even though they didn't actually activate the SIM, they still needed a valid IMEI to process the sale.)
Brought it home, stuck it in the newly unlocked phone, powered up, and basked in the slightly-weak-but-definitely-present signal. I decided that was good enough to commit another $10 to actual testing, so I activated the SIM through the website and signed up for the 10c/minute plan. I called up my wife's phone and walked around every corner of the apartment, and the signal held. Still more testing to be done, but so far things are looking good!
Even if it doesn't work out, it only cost me $10 to test the signal (could have been less if I was more patient), and another $10 to test the service proper. Not too shabby.