The Kingdom Hearts 3D Ultimania guide book is out today, and it has the expected interview with series mastermind Tetsuya Nomura.
A brief summary at FF Reunion reveals that the interview contains a teaser for the next entry (or entries) in the KH series. Nomura says that they've started making a variety of movements for the new title(s) in the series. He believes they'll be able to make an announcement pretty soon.
--Does the "Secret Message Unlocked" in KH3D hint at KHIII?
Nomura: About that, I'll leave it to your imagination. Well, you'll get the answer for that later.
Nomura: Maybe in a few years, but no plans for it right now. The Osaka team is really busy. ＼(^o^)／
--Is BbS Volume 2 = BbS2?
Nomura: I used "A Fragmentary Passage" without regard to its context. What I mean is, there's a story for Volume Two, without using that scene. I can't say there isn't a possibility of making it, but there are no plans for it right now.
--The Mysterious KH = 3D?
We interrupted titles in 3D, and we haven't reopened that discussion yet.
Osaka team has multiple titles in development. No plans for a Final Mix version of KH3D.