IE9 Platform Previews have been out since March 2010, with a new version hitting the site every eight weeks more or less. Even after the release of Internet Explorer 9 Beta, in September 2010, this is still no different. The platform previews deliver new rendering updates so they are the most up to date version of implemented rendering engine functionalities. Platform previews are not a full browser with Back button and such. However, very useful for testing site rendering post-beta.
At PDC 2010 Keynote, the latest version of the Platform Preview, version 6 was announced.
As a developer, this is a great opportunity to get your sites ready for Internet Explorer 9:
Download Internet Explorer 9 Platform Preview 6.