So he factory reset them, backing them up first. Then, when he reactivated them, he swapped the backups. The one that did NOT exhibit the issue was restored from the backup of the problematic device, and vice versa. The problem "jumped devices," and now showed up on the device that didn't have the issue previously. Meanwhile, the problematic device was now battery-issue free.
Naturally, it's still only two devices, but it seems to point to the battery problem being software or configuration related.
Annoyingly, some have said that iOS 5.0.1 made their battery life worse, and it clearly did so with the developer's older iPhone 4. He said,
"As an aside, I think that iOS 5.0.1 has introduced the battery bug to my iPhone 4. Typically the handset would drop about 3 - 4% battery capacity overnight (around 7 hours). Since installing iOS 5.0.1 I’ve noticed a much bigger drop of around 15 - 20% with no change in how I’m using the handset. I’ll keep a closer eye on this over the next few days and see if the pattern holds true."
Keep an eye out for iOS 5.0.2, it would appear.