WordPress 3.0.2 is available and this is a mandatory security update for all previous WordPress versions.I think the most important updated security issue is about the Author levels.If you have authorized authors,then this security is very important for you.
Official wordpress blog says;
This maintenance release fixes a moderate security issue that could allow a malicious Author-level user to gain further access to the site, addresses a handful of bugs, and provides some additional security enhancements.
And also here is the list of the files revised
wp-includes/ms-files.php wp-includes/version.php wp-includes/comment.php wp-includes/functions.php wp-includes/load.php wp-includes/canonical.php wp-includes/capabilities.php readme.html wp-admin/includes/plugin.php wp-admin/includes/file.php wp-admin/includes/update-core.php wp-admin/plugins.php
And also there were some custom queries declared to WordPress team and all are fixed.You can see them all here