I'm betting heavily on its' announcement at E3, no matter which platform it is.
In answer to the OP, the KH2 secret video was of the PSP game, Birth By Sleep, which, as others have said, is a prequel. There is also a DS game called Kingdom Hearts 358/2 Days (pronounced "three-five-eight-over-two-days"), which takes place while Sora is asleep, between CoM and KH2. It stars Roxas, and chronicles what goes in with the Organization during that time.