Lol nope, I haven't seen it...You haven't seen the Re: Coded secret ending, have you?
When the heartless and the corresponding nobody (Ansem SoD and Xemnas) are destroyed, the original person is reborn => ergo Xehanort is back in action.
On one more side note: When a person's heart becomes a heartless that person doesn't die, it only goes dormant.
If the heartless is slain by a keyblade the original person can return.
In the rare case that a nobody was born both the heartless and nobody (which "occupies" the body and soul of the original person) need to be destroyed to bring the original person back.
Turning into a heartless =/= death.
Anyways thanks, that was enlightening. it made a theory/question come to my head (I'll post it).
Another question: What about Terra? What happened to his heart when Xehanort was divided (Ansem SoD and Xemnas)?