I think that including movie clips, animations would be very neat. It would have to be done in a way that doesn't interfere with the flow of the text. Maybe a single line of text (like a hyperlink) would expand a box and show the clip. It could of course greatly increase the size of the file. Just basic images wouldn't though. With some eReader formatted books, they do include the cover image, and iSilo can of course include images anywhere in the document.
I imagine that there would be quite a demand for 'Enhanced eBooks' if they were done well.