For those who have upgraded their wordpress to version 2.1.1 recently, please download the new version 2.1.2 and update it again. It was said that the previous version was hijacked by a hacker which embedded a malicious code to the download
This is a highly unusual situation and more information will forthcoming. Also check out the official annoucement for more info.
p/s: You should not be worried if you have only upgraded to Wordpress 2.1. If you upgraded immediately after the Wordpress 2.1.1 was released, you should be okay too. Only downloads within the last three or four days are affected.

