The Telegraph has quoted Doctor Who costume designer, Howard Burden, in regards to John Hurt's role in the anniversary special. While Prince Charles and Camilla, Duchess of Cornwall, visited the set, Burden told them that John Hurt was playing a "dark Doctor" and that "There was a gap between Paul McGann playing The Doctor and Christopher Eccleston, when we didn't see a regeneration, and John Hurt will fit into that gap. He is a past Doctor not a future Doctor." Read the full article in the Telegraph about the royal visit to the set here.
This is not anything that hasn't already been put out there. From what I've seen, it's been the most popular theory. What remains to be seen is if this is The Valeyard or a Valeyard type character that is the darker side of The Doctor come to life, or if John Hurt's Doctor is #9. Also, will elements of the Time War be involved in the 50th? I think it would be well received if so. It will be interesting seeing how this revelation steers the rumor mill.