I don't think Zelda will have downloadable content. Read the follwing questions and answers from this article again:
Seems to me that Harrison makes clear that Zelda does not have any downloads (to his knowledge) but other titles might/will, interpet it how you likeGP: How long is Twilight Princess?
Harrison: There are 70+ hours of gameplay -- it's significantly longer than Windwaker.
GP: What sorts of online features are you looking forward to in the future?
Harrison: Downloadable content, such as additional levels.
GP: Any plans for any type of post-release online component for Twilight Princess?
Harrison: I haven't heard of anything yet. I think everyone is still busy trying to make sure the game gets out correctly.