Remember the part in Goblet of Fire when Harry thought he saw a 'gleam of triumph' in Dumbledore's eye when he recounted to DD about how Voldie had used Harry's blood to regenerate?
I think I finally understand why DD would have had this 'gleam'.
I believe when Voldie was defeated the first time (by Harry as a baby), the remaining Horcrux inside of Voldie was damaged beyond repair. When he regenerated in GOF, he then used the Horcrux he had implanted in Harry to regenerate - that is why he needed Harry's blood.
If this is the case, then, even though Harry was the seventh Horcrux, Voldie has now taken that Horcrux back into himself, and Harry is now possibly Horcrux-free. Meaning that although Harry will *think* he needs to sacrifice himself to defeat Voldie, believing that he himself is the last Horcrux, he will not have to do so, because the Horcrux that was in him has now been taken back by Voldie.
So what this would lead to is a dramatic ending to the series, with Harry on what he believes is a suicide mission, only to find that he is spared because he no longer contains the Horcrux in him that Voldie implanted.
That's my theory as it stands for the moment, anyway!