Audible has arguably been more successful at promoting and publishing audio books than anybody has been at promoting and publishing eBooks. Correction: that is not even arguable. Audible has been really successful with audiobooks and nobody has yet been really successful with eBooks. It is certainly intriguing that Amazon, with its own Kindle, will be able to pipe audio and ebook through the same device. Apple has had an effective relationship with Audible, and they may not be too pleased to see Amazon as the new owner.
An Audible overlay should go well with the Kindle, but this is making Amazon very committed to the download model of eBook distribution. Apple, on the other hand, has been making moves in the direction of a streaming/access model for film distribution. Google Book Search is at the streaming/access end of the spectrum. So who is going to get this right? If Amazon, Google and Apple are going to have a tussle over books, they are coming at it from different angles. Which certainly makes life interesting.