From: http://blogs.adobe.com/digitaleditions/2010/01/epub_ipad_and_content_interope.html

“Although Apple has standardized on the EPUB format, because it employs its own DRM to protect eBooks consumers will lose out on much of the benefit of an interoperable format simply because they won’t be able to transfer content across devices.

For example, EPUB content protected with Apple DRM won’t work on numerous eReaders like the Barnes & Noble nook and the Sony Reader, not to mention future, forthcoming models. Similarly, protected EPUB eBooks obtained from thousands of online booksellers (including Barnes & Noble) and most public libraries (including The New York Public Library)—are unreadable on the iPad.”