They might put it on the ps3, but if you watched the secret ending it had a video, so obviosly they have the technology on the ps2 to make that game for the ps2. Also they will probably release a new gta for the ps3 as the launch, not very many people will buy a ps3 just for kh3. Chances are it will be on the ps2, but it will work on the ps3.
If you love coheed and cambria, go to this forum and join. Promote it wherever you can
http://cambriaholics.20.forumer.com/index.php?sid=ca43f68f3de8302ffe9996b74957e682