As Matt mentioned, it is a security update. You have to update your WordPress installation as soon as possible.
I did it when I received the email and read the message from Matt.
Version 3.0.4 of WordPress, available immediately through the update page in your dashboard or for download here, is a very important update to apply to your sites as soon as possible because it fixes a core security bug in our HTML sanitation library, called KSES. I would rate this release as “critical.”
Security is the top issue of all kinds of web sites, including weblog or blog.