I can't remember if it ever was confirmed, but I know for a fact that it was heavily, heavily implied, which pretty much means it's confirmed.
When a Heartless is defeated, its heart disappears into a state of suspension in another Realm/dimension that we don't know of yet. When a Nobody is defeated, its body rejoins with its heart. If a Nobody gets defeated first, there's a smaller chance that the heart and body+soul will rejoin, but it's still possible.
i personally, even though there is absolutely NO basis for it, believe that axels heartless has already been defeated. imo, its the trickmaster, or whatever tht thing in wonderland was. i mean, it controlled fire, its a horribly annoying boss to fight in the games its in, that just goes with axels personality. still, no basis.
on topic, i see riku interacting with disney characters in a way where he has a guilt trip. because i believe the PoH worlds are gonna be here, at least some of them. when he sees any of them, he will feel guilt. and im sure he actually personally got a few of them, so it should be interesting.