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.
I agree with you personally, since it's the 50th Anniversary and I think they have bigger fish to fry than stuff that happened in 2008 with the new Daleks. It would make more sense to delve deeper in the mythology than that.
However, it's still interesting stuff to ponder. Him being the clone would dovetail with the presence of Ten and Rose in the special, and tonight Eleven said "I said he's me. I never said he's the Doctor." It would also be less complicated than a forgotten regeneration tying the fandom in knots....