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 remember back when "Journey's End" aired, I thought the Meta-Criss Doctor was the way they would give us the Valeyard. You know, since the Valeyard is supposed be between the 12th and 13th Doctor, meaning he was supposed to be Doctor 12.5, and since the Meta-Crisis Doctor is technically Doctor 10.5, it would make sense that the Valeyard is a regeneration of the Meta-Crisis Doctor. And before anyone says, "He's human and can't regenerate!" If the Master can't get more regeneration, and River Song can regenerate, surely a human with the memories of a Timelord can figure out how to regenerate.
Christ, it's gonna be a long wait to the fiftieth.