Black 2 & White 2 can work with 3DS apps (as shown by the Dream Radar), so you can deposit Pokémon from Black 2 & White 2 in the Box app and then withdraw them from the Box app using Pokémon X & Y. A quick and easy way to transfer your Pokémon.
I see no reason it couldn't be compatible with older-gen games; the Dream Radar was. And Dragon Quest Monsters did something very similar to this, as well. So yeah, this is what I've been expecting for a while.
Bonus points to GameFreak if it's compatible with the fourth gen!
Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere