Please, please, please go and upgrade your blogs right now if you are using WordPress 2.1.1. As Matt outlines, a hacker managed to gain access to the Automattic server with the file downloads and modified core files. The entire 2.1.1 version has been declared compromised and unsafe. Please heed this warning and go download 2.1.2 right now.
Update: Please direct all questions regarding this breach to firstname.lastname@example.org.
That is all.