If you updated WordPress in the past few days, download the new version and update it again. Someone was able to break in and malicious code was added to the download. The new version is WordPress 2.1.2.
For more information see: WordPress 2.1.1 dangerous, Upgrade to 2.1.2.
It appears that this is a mandatory upgrade for everyone, whether you upgraded previously or not. The security patches in this and the last few versions are necessary to protect your site.
This is a highly unusual situation and more information will be forthcoming. Check out the announcement for more details.