Kingdom Hearts has always been a game that defines the era. It is a technical achievement. I predict that the Wii will get a sidestory - and thats not hard to imagine since we have three different stories / eras that are interwined (Sora, Roxas, and the Future Keyblade War world);
BUT THE MAIN FRANCHISE WILL BE ON THE PLAYSTATION 3.
This strategy satisfies Wii owners looking for a more casual game and Playstation 3 owners who would forgo the card / puzzle game for the full head RPG experience. By the time the game is released the Playstation 3 will be hovering at $250 dollars anyhow.
People take for granted that Sony and Disney have a very close relationship thanks to the Bluray consortum and that Sony owns around 13% of Square Enix.
Kingdom Hearts has never been technically impressive. Sony owns closer to 7ish% of Square right now, and that didn't stop Dragon Quest from swapping platforms and it is as big a series roughly. And I don't think Sony has much leverage with Disney due to Blu-ray. disney is on pretty good terms with all the console manufacturers right now, and wouldn't see much improvement in their Blu-ray division because of a deal with Sony over a video game.