On Wednesday WordPress released a update they deemed important enough to have the “critical” title. The update fixes a security bug in their core HTML sanitation library. This fix takes WordPress to version 3.0.4 so if you aren’t already running it, you’ll want to update immediately immediately!
You can download the security update at http://wordpress.org/download/ or from your dashboard.
And for a more technical perspective you can checkout WordPress’s release notes here.