This makes a lot of sense, in the wake of certain publishers who have released their book publications as iPhone apps, rather than as PDFs, Kindle Books (.azw format), or .mobi (MobiPocket format). The advantage of this for Apple is that it leverages their current iPhone/iTunes Store momentum and infrastructure, while maintaining ease-of-use for customers. It does, however, mean more VHS/Betamax format wars for consumers.
Right now, it looks like Mobipocket and other formats, used by open-book readers like Sony, the Nook, Philips', and others, are fighting an uphill battle, at least in the U.S. The Big 3 in the electronic publishing industry are PDF, MobiPocket, Kindle Books. But even though an iPhone app is not really a file format, is is a method of delivery, which is ultimately what the file formats allow. Amazon and the iTunes Store offer convenience of delivery, sync, and portability, while PDFs offer print options and ubiquity due to a 15-year head start. Since Amazon purchased Stanza, buying books from the Fictionwise book store has become more disjointed and harder to do. IPhone app-books, in contrast, are becoming easier and easier to download. Another advantage for iTS users is that they can purchase their audiobooks from the same place, to the same devices. Amazon's Kindle does "read" certain books to you, but reviews of the auto-reader are mixed.
No doubt Android (Google's smartphone OS) will be incorporating app-books into it's store soon, if it has not already. That, together with Google's recent eBook moves and it's ginormous search-engine audience, could be the beginning of a slow death of Amazon's initial raison d'etre. I foresee an Apple vs. Google fight to the death in most consumer information markets, including eBooks.
Good thing Amazon has leveraged it's cloud computing and fulfillment infrastructure into separate businesses. Wonder what Microsoft is up to?