It's my understanding that Apple is supporting a _subset_ of the ePub standard, as are most of the readers. I don't know of _any_ reader that supports the whole standard. If you do, please point it out as I would love to play with it.

To my knowledge Apple isn't extending the standard at all. Is Apple _adding_ anything to the standard? Isn't the DRM (grrrrrrrrrr) that Apple's adding covered under the standard?