The new issue of Game Informer has this quote from Tetsuya Nomura:
"This particular game came into existence because we wanted to do something for the North American fans, so we'll be announcing it at E3."
The game in question is Kingdom Hearts! With the release of 358/2 Days serving as a side-story and Birth By Sleep being released this September being a prequel, I think we can safely assume that they ARE GOING TO ANNOUNCE KINGDOM HEARTS III!
It is due time for the main series to continue and it fits nicely within the 4 year gap that Squeenix usually takes to release their games. I can't wait to hear more details about my new favorite game ever!
They could always make another spin-off though. Please, please don't let it be another spin-off.