I don't think they've made a decision yet at all, but are just taking a "wait and see" approach.
If FFXIII comes out and sells like 4:1 on the PS3 (not saying this will happen) and the PS3 Userbase is booming (compared to the 360) then I think it will stay Exclusive.
If the PS3 version outsells the 360 version 1.5:1 and the userbases are still close in EU/NA, then it probably would go Multi.
But since this is all 2+ years away, a lot can happen.
4:1... it'd need to be like 10:1. The estimated price for porting a game from one HD console to the other is 10%.
So anything over 10:1 is pure profit for the 360 compared to PS3.