An update for the Kindle Fire has started rolling out to users. Version 6.3 doesn't add anything revolutionary, but it includes several neat extras, including a toolbar for sharing notes and passages from within books, as well as "Book Extras," which adds the written equivalent of DVD special features like character biographies or a glossary to books. Personal documents added to Kindle will now be synced to the Amazon Cloud, and textbooks will be able to preserve original formatting and page numbers. You'll also get more time to watch movies: the rental period now starts when you begin watching, not when the movie is downloaded.
As some people have noted, installing the update will break rooting on the Fire. However, the xda-developers forums have already put out an update that gives you root access. If you haven't already been prompted to update your Fire, you can get the file from Amazon's site.