This counts to both this game and HeartGold!
Will Celebi be available in the game?
It would make perfect sense as it is the only Pokemon that has not been obtainable world-wide yet. it was only available to be accessed via AR code.
In S/G/C, it was available in Japan through the phone function they created. It would give you the GS Ball and you could catch Celebi!
This is my theory:
Nintendo will create a GS Ball Event that allows you to access Celebi. Like the Member's Card=Darkrai and Azure Flute=Arceus event functions.