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Some people supposedly got asked to download when they go to the app store. I haven't seen it happen though.
 
Just to provide some further explanation, iBooks is not an included app with iOS 4.0. As other have stated, it is a free app available in the app store once iOS 4.0 is released.
 
Developers with a developer account can download it. All you do is download a book and it downloads the app with it.
 
Oh, so iBooks is coming today? Ace, be great to sync with my iPad. As a 3G owner (till Thursday) os4 is distinctly underwhelming, but iBooks will make it more worthwhile.
 
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