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Do not watch this video until you've seen The Name of The Doctor -- this is the final moments of that episode!
After watching this a few more times, I'm less and less convinced that Hurt could be the Valeyard, or really anyone from the Doctor's future. The Doctor says that Hurt is his secret. Which, while of course we all know everything about the Doctor is timey-wimey, all but necessitates he's from the Doctor's past, something the Doctor has been running from for a long time.
The Valeyard, however, is either the Doctor's twelfth incarnation (the Master at one point called him the Doctor's penultimate incarnation), or an amalgamation of the Doctor's twelfth and thirteenth incarnations. Either way, though, that's from the Doctor's future, and while the Doctor did indeed meet the Valeyard, and the Valeyard was more than a little bit of a dick, nothing he did I don't think qualifies him to be some dark "secret" of the Doctor's.
I'm also now 100% convinced that Hurt is not going to be Twelve. Why? Because we've already met him, he's going to be in the 50th Anniversary special in November, but Smith is holding onto the reins through the Christmas special and into season 8 next year -- there might be a mid-season regeneration, but even so I don't think they could hold onto Hurt for that long and still count on him being willing to become the Doctor.
So, I'm more strongly leaning toward the two other theories: He's either the Doctor who fought in the Time War (or at the very least the one who used the Moment to end it), be that an older McGann/Eight or the real Nine, or he's the Doctor before Hartnell/One, who did something terrible back then on Gallifrey that then prompted the Doctor to take on the name and to seek redemption.
My favored theory right now is the Time War one.
A new theory that I came across, though, is also rather intriguing: Hurt could be the Doctor from any point in his life, including Smith/Eleven's time! At the point that we meet Hurt, we're essentially inside the Doctor's head; whatever Hurt did, the Doctor clearly does not associate with who he is. Thus Hurt could be a disassociated version of the Doctor, the Doctor trying to literally distance himself from something terrible that he's done.
I think it's interesting, but the previous two theories I think are still stronger.