So, I have already brought this up on a few comments before, but since those are largely missed, I'll post this here, so people can see it better.
Back when Kingdom Hearts Final Mix + was released, there were a few special editions, that contained a book, where Nomura revealed plenty of juicy bits about the game canon. The one I want you to take a look at, is the secret report, where Nomura, among other things, talks about the hearts. It is the Director's Secret Report XIII. You may find it's translation
here at gamefaqs. Now take a close look at this particular tid bit.
VIII - When the heartless are defeated, what becomes of the stolen hearts? Also, when members of Organization XIII and other Nobodies are defeated, do they return to their original form?
When heartless are defeated, essentially the owner's hearts are rejoined with their once extinguished body, whichever world it may be on. As for the whereabouts of hearts in KHII that turn up, this time they remarkably went to the organization (there is a foot note here that says "In Kingdom Hearts II after heartless were defeated, the many hearts were absorbed by the Kingdom Hearts of "people's hearts"). However, in the rare case that the body changed into a Nobody, when there is no container for the heart it resorts to a state of suspension.
In the case of a Nobody being defeated, it's a little more complicated. If the above mentioned hearts are liberated, they return to their original form. However, if the heart is still stolen by the heartless, the Nobody's body is swallowed by darkness. If somewhere in the world their hearts are taken back, perhaps they might be able to return to their original human form.
Emphasis on the answer is mine.
What does that means, you ask? Well for one, it meant that Xemnas was full of shit. And, like many people have theorised here, he may very well know it, since he studied the heart far more than Ansem did.
This also means that Sora's actions in both games, allowed Xehanort to come back to life. *watches the plunnies running around*
Just one more thing to add fuel to the fire of the discussion that started a few rants down.