I guess, there will be some exclusive DSi features like downloadalbe content or similar stuff. But I don't think, a DSi will be mandatory.

Well, and like I thought, they're releasing another DS Zelda before the next big Wii Zelda deal. Phantom Hourglass engine works fine and a creating new a game for it is done much faster than a completely new game for the Wii.

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Hyrule Blog