Disclaimer: I'm not muscling in on JK's turf - just gambolling on it, like a spring lamb, having fun working out the literary and psychological puzzles which she is having fun setting us.
TURN AND TURN ABOUT
The green blast caught the boy in the chest as he arced, fell, died. Undied. Opened green eyes as Tom staggered / opened Voldemort's crimson eyes, opened his mouth on hissing words which spoke light. "Get up, Tom," said Harry's words in Tom's mouth. "Learn to be sane."
"But how?" he said, staring at the hospital ceiling. "How did I control him - how did I survive him? Again."
Hermione patted his hand. "You know Voldemort meant to make a Horcrux by killing you, as a baby? Well, it wasn't just Avada Kedavra which rebounded. You weren't his Horcrux: he was yours."
I wanted to get this posted as a concept before Deathly Hallows comes out, so that if this proves to be canon people will know I thought of it myself, and didn't just get it from the book...
Of course, it raises some problems: it would mean that a fragment of Harry's soul was missing, or at least operating by remote control. But the fact that reborn!Tom is so snake-like, and Albus thought Nagini was a Horcrux, suggests that when a living thing is made into a Horcrux the creature, and probably the soul-fragment inside it, remain closely linked to the progenitor.
So whether Harry was Tom's Horcrux or vice versa there would still be something of Tom in Harry (his wild hatreds, the sudden inexplicable urge which made him look in the Pensieve etc.), and something of Harry in Tom, which could be called to, until Harry's love overcame Tom's coldness. And that two-way flow might mean that Harry was still in touch with his sundered bit of soul, and so not made blurred and incomplete as Tom was when he put his soul into non-living objects.