It's already been confirmed that Kingdom Hearts III will not be the next Kingdom Hearts game either. So you have at least one more "spin-off" to go. Really though, for all intentional purposes Kingdom Hearts Birth by Sleep kinda works as Kingdom Hearts III. Square Enix could have called it KHIII if they wanted and nobody would have complained a lot like Kojima could have simply called MGS: Peace Walker MGS5.
As far as an appropriately titled Kingdom Hearts III though, that won't be even entering full development untill Final Fantasy Versus XIII is finished and released. If Wada's recent statement on Square Enix's major releases for 2010 is any indication, FFvXIII won't be coming this year either. Which means if Versus is a 2011 release, then KHIII won't start full development until then which means a 2012 release at the earliest, but much more likely a 2013 release.







