from the announcement:
Today we're pleased to announce open enrollment into the Kindle developer beta program. Read more about the program here and sign up here. We'll ask you to briefly describe your idea during the enrollment process. Enrollees receive information on how to download the development kit, how to receive support from us while you develop your project, and how to submit your finished project to the Kindle Store. The development kit includes a Kindle simulator (both 6" and 9.7") that works on Windows, Mac OS, and Linux. You can also register up to three Kindle devices through the developer portal; the owners of those devices will receive invitations to download and test your active content.