I had been attacking the treadmill in the gym strenuously enough that when I came out I was still pretty warm. So, along with my gym bag, I put my winter coat into the trunk. It wasn't until I closed the "hatch" that I realized the fob was in that coat, not my sport coat. Although the body is mostly fiberglass, evidently the trunk compartment isn't, so the radio waves didn't reach the receiver and the trunk release button wouldn't work.
After pounding my forehead a few times, I remembered that one of OnStar's features is that they can remotely unlock the doors. Fortunately, my cell phone was in my pocket, not in the coat in the car, and I had previously programmed in their phone number, so I gave them a call. After a few questions to verify that I was not actually a car thief, they sent a signal to unlock the doors.
It was really eerie – there was no sound, but the lights blinked and I could now open the door, and from inside I could open the trunk, retrieve the fob and drive away.