Recent developments seem to show that Amazon is moving away from a tight bonding between the Kindle book formats and the Kindle reader - the new Kindle DX features support for PDF files, rather than just Mobipocket and AZW, and the new 'Kindle for iPhone' program lets Kindle book buyers use the iPhone as their reading device if they prefer. Indeed, Jeff Bezos has now explicitly said that Kindle books and Reader are two separate businesses:
The device team has the job of making the most remarkable purpose-built reading device in the world. We are going to give the device team competition. We will make Kindle books, at the same $9.99 price points, available on the iPhone, and other mobile devices and other computing devices
Can't help but feel that the unexpected success of the iPhone as a reading device, using third-party apps, has swung Amazon's strategy somewhat...