For a while the theory has been out there as to whom John Hurt is playing in the Doctor Who 50th Anniversary Special. The popular one is that he would be playing the 9th incarnation of the Doctor, some type of alternate universe version of him.
This appears to be true, at least according to Hurt himself.
The following has been pulled directly from The Sun's website:
"A source explained to TV Biz: “John will play the real Ninth Doctor. Christopher Eccleston has always been thought of as the Ninth Doctor but now that John has been revealed as the Ninth Doctor, it shuffles Eccleston’s Doctor to be the Tenth Time Lord, Tennant the 11th and Smith the 12th."
“The reason the Doctor has forgotten his ninth incarnation will become clear but it’s to do with the Time War and his shame over his behaviour in it."
"Eagle-eyed fans have spotted that John Hurt’s costume bears a similarity to Paul McGann and Christopher Eccleston’s — his version would have come between them."
Go check out the article on The Sun's website for the full scope!
http://www.thesun.co.uk/sol/homepage/showbiz/tv/4921644/doctor-who-50th-anniversary-special-spoilers.html