We’re happy to announce that a new version of WordPress is now available for download. This set of improvements and security fixes is in line with our commitment to maintaining an extremely stable 1.5 series. In addition to fixing a number of bugs and adding requested enhancements for plugin authors, this release also addresses all of the security issues that have been circulating the past few days. This is a very straight-forward upgrade, you just need to upload the new files over the old, but as always the Codex is the best resource on upgrading.
P.S. If you upgraded late Sunday night double-check that you have the latest version of wp-settings.php on your blog.