WordPress version 3.04 has been made available as a security release. This means it is very important to update your WordPress blog to this version as soon as possible because it fixes a core security bug in the HTML sanitation library, called KSES.
According to WordPress, this update is rated critical and the security issue affects all versions of WordPress prior to 3.0.4, so if you are still on a 2.X release you need to update as well.