Update, 25 May 2011: I have recently published a new miniguide explaining exactly how to add audio and video files to your EPUB formatted ebooks so that they work properly and look great in iPad, iPhone, and iPod touch, as well as NOOK Color. You can find more information in this article.
Simon and Schuster released the Enhanced edition of Nixonland, by Rick Perlstein, today. This enhanced edition includes both video and audio snippets. Those of us interested in creating our own ebooks with embedded multimedia have been trying for some time to figure out how to do such a thing AND pass EpubCheck 1.0.5, ostensibly one of the conditions for making it into Apple's iBookstore.
Curiously, though, a look inside Nixonland reveals that this document is not XHTML 1.1 because it uses HTML5's <video> element to embed video, and thus cannot possibly pass EpubCheck 1.0.5.
Either Apple made a concession to Simon and Schuster (most probable) or it suddenly is not requiring EpubCheck validation.
I will be posting more specifics within the day.