He could also be the Paul Mcgann doctor, or a younger version of the first doctor.
this would be stupid. Why not just hire Paul McGann? He's been very involved in a lot of Doctor who audios, etc off of television, and would be more than willing to play an older version of himself. And he can't be a younger first doctor, because he himself is actually 15 years older than William hartnell when he portrayed the doctor anyway!
Instead, there's this theory, which is most plausible:
The most common theory I e heard, which is more in depth than what SFK said, is that John Hurt is playing the REAL 9th doctor (making all other doctors +1), the one who took place at the Time War. This is backed by the fact that the Doctor said he was not "the Doctor", he didn't work under the name "the Doctor," when in "Journey to the Centre of the TARDIS" Clara reads the doctors name out of the book of the time war, when it gives his real name and not "the Doctor." Why would that be? If you've seen older episodes before the reboot of the doctor on Gallifrey, the other timelords call him the doctor. Why then, would his name be recorded as his real name in the history of the time war?
Think about it.