Given some of RTD's comments I'm sort of wondering if this is perhaps an alternate universe, perhaps related somehow to "Turn left" perhaps; where the events of Stolen Earth/Journey's End and the specials never happened, so Donna never had her memory suppressed and the Tenth didn't regenerate. Maybe perhaps Neil Patrick Harris is a minion of the Trickster, or the Trickster himself? I know a "Degeneration" is the big theory, but this kind of just occurred to me.