I have a theory that the "Dusks" seen in the KH2 previews ( I do not know if that is their name, I just saw them referred to as thus on one of these threads) are made when the body and Heart of a heartless is torn apart ( i.e. when they are killed) and that the nobodies are humans who became heartless, but regained their hearts. Therefore, they are the Nobodies of the heartless.