Just got off the phone with AppleCare. The senior tech I spoke with said he had personally heard of one other incident in which this happened. He told me to check my HDMI cable (which has always worked before). He said the ATV2 only outputs at 720, so he wasn't sure what would cause it to output at a different resolution. He said if it happens a second time I should contact Apple and they'll replace it.
In other words, I'm too confident Apple knows exactly what the problems is, but the impression I got was that they believe it may be a hardware issue if they're going to replace the unit. I told him that I had read on the internet (MacRumors and elsewhere) of several other people having the same problem. If it happens a second time I do plan on getting a replacement ATV2 in case it is a hardware issue.