Yes, I agree, the thing in the corner at King's Cross was Voldermort's soul. Harry told Voldermort during their face off that he had seen what would happen to him if he couldn't show remorse.
When Hermoine first told Harry and Ron that remorse was the only way to "undo" a horcrux, I thought that Harry had probably become a Horcrux, but that it had been undone the night in the ministry when Voldermort had inhabited Harry's body. Dumbledore had talked about that act being unbearable to Voldermort because the love Harry was capable of feeling was excruiatingly painful to him. I thought, for those few moments, Voldermort had been forced to "feel what he had done" (loosely quoting Hermoine's description of what it would take to undo a horcrux).
That being said, I was a little confused by the way Roling *did* choose to save Harry... Dumbledore said it was because of the protective spell that had been cast on Harry by his mother's sacrifice.... "He took your blood believing it would strengthen him. He took into his body a tiny part of the enchantment your mother laid upon you when she died for you. His body keeps her sacrifice alive, and while that enchantment survives, so you you and so does Voldermort's one last hope for himself."
But I am confused by that... I thought the enchantment was supposed to break when Harry turned 17?
Another question is... Harry told Voldermort that his last chance to save himself was to show remorse before he died (because the remorse can undo a horcrux). But... you can't undo a horcrux after it has been destroyed can you? Harry and co. had already destroyed the horcruxes, already killed those little pieces of Voldermort's soul.
Also, why did Harry's scar start hurting again in this book? Had Voldermort stopped using occlumency? Did I miss something?
I know, lots of questions. Overall, I loved it. I always thought Doby was an utterly lovable character, but had to love him even more for his willing sacrifice. I love that Harry's sacrifice cast a similar protective spell over "Dumbledore's Army", and that Neville got to pull the sword from the hat. Such a compelling, and to me, ultimately satisfying story.