Apparently
Orla Brady (who played Walter's wife and Peter's mum on Fringe) is to play someone from the doctor's past in the christmas special.
Several people speculating (at least in the comments section ) that she could be playing Romana, or The Rani.
---------- Post added at 14:04 ---------- Previous post was at 13:48 ----------
Also, the mirror has a story that Matt Smith is actually the 13th doctor (apparently John Hurt's regeneration does count, and David Tennant is counted as both 11 and 12). Apparently the regeneration limit will be addressed in the Xmas episodes..
http://www.mirror.co.uk/tv/tv-news/d...s-matt-2847509