Lightning was in the "Michael" trailer so Sony clearly has some kind of deal or lisence worked out with SE, but whether Cloud gets in depend on whether or not the lisence is just specifically for Cloud or full access to Final Fantasy in general.
The more I think about it, the more I see something like this happening. It'd be easy money to those who do want a variety of Final Fantasy/Kingdom Hearts characters in the game. Just add them as DLC and make it $10. Easy money.