A theory I just read on Youtube....Hurt might be the meta-crisis Doctor that stayed with Rose and aged. Although Ten was angry at his clone at the end of Journey's End for killing all the Daleks, I don't know if he'd be mad enough to block him out.
Edit: I just looked at it again, and Hurt's haircut is similar to Tennant's, though missing the telltale sideburns. I'm still leaning toward the side that he's a hidden regeneration, but the clone thing is interesting.
I'd say it's far more likely he'll end up being a regeneration between 8 and 9 or being a pre-Hartnell Doctor than being the metacrisis Doctor. This is a part of the Prime Doctor, just a part that's been buried. Whatever he did, the Doctor buried that part of himself, hid it, and considers it a broken promise so terrible as it to be his darkest secret. It's gotta be bigger than locking away Gallifrey and the Daleks.