[spoilers] John Hurt is the Doctor? then who the hell is Matt Smith? or ...? [/spoilers]
Some people are saying its The Valeyard which is a classic Doctor Who villian basically being an evil future incarnation of the Doctor. Or some say he is the Doctor that basically killed or let all the Time Lords be killed. Either way its probably a future, past, or parallel universe version of the Doctor and because its in the strand of his life he can obviously meet him.