OP here. I haven't had this recur, but I've also fallen into the habit of plugging in the iPad before I go to bed. Are you all experiencing this with the iPad plugged or unplugged?
I mentioned this to an Apple store person on Monday, was unsurprised to hear she hadn't heard of it at that time, she said what they usually say "it'll probable be a software problem"...and then I forgot about it because it never happened again, until this thread start getting bumped.
Maybe I'll try to leave it unplugged tonight and see if I can re-create the problem consistently.