The Guys at io9 have some ideas about this weeks Doctor Who...
1) The Doctor has a failed regeneration, with his severed hand involved somehow, and as a result he turns into two different Doctors, both played by David Tennant. (We showed some pics of Tennant wearing two different colored suits on the beach where he said goodbye to Rose in season two.)
2) The Doctor has a failed regeneration, which splits him into two Doctors, one played by David Tennant (thanks to the severed hand?) and the other played by David Morrissey (who's in the Christmas special later this year playing a character referred to as "the other Doctor" on the actors' trailers.)
For more of their theories visit their site above.