Heartstation.org reports that the latest issue of Dengeki PS contained an interview with Kingdom Hearts director Tetsuya Nomura which not only mentioned Kingdom Hearts 3, but how Kingdom Hearts Birth by Sleep for PSP will relate to it. The article also lists the release date for new chapters of Kingdom Hearts Coded for mobile phones will begin distributing June 3 2009 in Japan. They’ve also posted scans containing new images of Kingdom Hearts Coded and Kingdom Hearts 358/2 Days for Nintendo DS
http://news.zergwatch.com/2009/05/07/kh3-the-return-of-sora scan/
I also read in another article, where they say that once the production off all 3 spin-offs are done, they will start with Kingdom Hearts 3. It's also interesting that it will relate to Birth by Sleep.