Please consider this an open entry to all Nobodies that Xemnas has extended his assistance to. I will attempt to address questions or concerns to the best of my knowledge.

First and formost, to the issue of the hearts promised: Obviously the scenario didn't play out exactly the way it was expected to. I have no specific explanation for what DID happen, but as it stands, Xemnas is no longer with us for the time being. The best we can deduce (at least, according to Vexen) is that he was... 'displaced' by the memories he was attempting to recover, resulting in the reintroduction of an older persona: the man who called himself Terra.
I realize this puts Xemnas's promise into question for many, if not everyone else. Be assured, however, that Terra has offered his help on the matter. Unfortunately, in speaking with him on the matter, it has become clear that we can't expect the easy solution we'd been hoping for. While it is possible (necessary even) to use a keyblade to return a lost heart, Terra is uncertain that he himself will be able to do it. Moreover, it's required to locate each individual heart, specifically the heartless that still retains it. For those that are unable to locate their heartless or have had theirs since destroyed, it's doubtful anything can be done.

Until further notice, Terra will be staying on the World that Never Was. Anyone wishing to know more may speak with myself or Vexen for now.