It's probably going to be obtainable through one of these three:
A: It's a Wi-fi event (Most likely). You get it via Wi-Fi at Toys R Us or from plain Wi-Fi at your house like with Shaymin or the Rotom event.
B: It's a post game legendary, kind of like Cresselia. There is a problem and a Celebi wing is the only way it can be solved and Celebi flies around Johto.
C: It saves the day when all hope is lost and Lugia and Ho-Oh are in the midst of fighting or whatever. Obviously, no one knows what the climax of the plot will be for this game but if they do include Celebi in the plot it will probably be the force that saves the world from Lugia and Ho-Oh (assuming they will be against each other).