Whatever your opinion about Al Gore, check out what Mike Matas and the designers at Pushpop Press have done.
This is amazing stuff, and Pushpop aims to make the platform scalable so that costs of publishing truly interactive content in an ebook form are kept manageable.
This isn't about choosing a plot line. This is about embedding video, audio, graphics, and data into an ebook.
I love the access to underlying data in charts and graphs, but what's really fun is how you can sort the data. Then, when you want to return to the text, you just "fold" the graphic in half and put it away.
This is really starting to feel like a space race between Apple and Amazon. When the iPad offers this kind of interactive technology plus the ability to toggle to a simple e-ink option for straightforward text, I think they'll have won.