Nate, you don't have to take my word for this. Get an iBooks fixed-layout book and run it through the ePub validator at
http://validator.idpf.org/. It will fail validation - I guarantee it.
Regardless of the fact that some people may choose to call such fixed layout books "ePub" (and it's certainly true that some people do), it's just plain wrong to do so. They are not valid ePub books; they are books that can only be read by iBooks.