I've seen rumours that Smith isn't actually in the Christmas special. The regeneration happens at the end of the fiftieth. Which would mean Smith wouldn't have to wear a wig and ties in with -
Ben Stephenson, Controller, BBC Drama Commissioning: We can't wait to premiere his unique take on the Doctor on Christmas Day and we are sure he's going to become one of the all-time classic Doctors."
A ten second appearance is hardly a premiere which is what would happen in a regen episode. Unless of course Smith falls off his exercise bike and regenerates at the start of the episode.
