Interesting point. My thoughts would be to use it to create a more interactive version of a more classic pdf (distributed on other sites).

The iBooks format allows for the inclusion of a couple of nice things. The interactive graphic widget allows you to tag an image. Clicking the tags zooms to a predefined view. It also allows for the inclusion of video. Are videos allowed in pdfs these days?

I know that apple's gone well beyond the epub format by allowing javascript in there. I'm curious as to how much added power that could throw into ebooks - though obviously it opens the door for some truly egregious abuses of the ebook format at the same time!