Spacesamurai, a MacRumors forum member says, "I am in Japan so it is 2013, and I am finding that my iOS devices (iPhone 4 and iPad 2) are showing "Do Not Disturb" even though it is outside of the time I set for them. Not sure if this is related to the new year. Reloading the devices does not help and the software is up-to-date."
Interestingly, not everyone is reporting the issue. If you are, turning the setting off then back on appears to resolve the problem.