Aggregated daily data usage
I just got off the phone with an iPhone tech support agent, and he said (after consulting his superior) that AT&T had probably posted my data usage for the day as a single total, which is why I had a 288 MB data usage entry around midnight recently.
I have Push notifications for email enabled, but it's set to Fetch manually (so it doesn't automatically download more than about 2 lines of the email... no huge data usage there).
My apps have to be manually updated (requiring me to type in my Apple password), so it wasn't any app downloading an update in the middle of the night.
The only video I had watched on my iPhone around that time (the 288 MB usage was supposedly on 12/26 at 11:43 p.m.) wasn't until the next day (about 11 hours later), and AT&T specifically told me that, once a data usage is added to the log, it should be at the right day/time. It might take up to 72 hours to be added to the log, but once it's added it should show the correct day/time of the usage.
Assuming the 288 MB was simply a total of 12/26's data usage, I still haven't figured out what it might have been.
Just wanted to share my experience in case it helps anyone.