bbPress 0.9.0.2 is available for immediate download. This release includes some small changes and fixes a few bugs, but is first and foremost a security release. The vulnerability is not yet publicly known, but will be soon.
Many thanks go out to Steven J. Murdoch for identifying the issue (CVE-2008-1930) and working with us and WordPress to resolve it.
Note: Those of you with integrated WordPress/bbPress installs should upgrade WordPress first and then will probably have to update the “WordPress database secret” setting in your Settings -> WordPress Integration: Cookies admin panel after upgrading bbPress.