We've Been Hacked

You can start working your way through these resources:
http://codex.ww.wp.xz.cn/FAQ_My_site_was_hacked
http://ww.wp.xz.cn/support/topic/268083#post-1065779
http://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
http://ottopress.com/2009/hacked-wordpress-backdoors/

Additional Resources:
http://sitecheck.sucuri.net/scanner/
http://www.unmaskparasites.com/
http://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html

Use a FTP client and delete all files, or, if you want, download everything first. Then reinstall WP from a fresh download. Do the same with your plugins and themes. Do not upload any of the eventually dowloaded theme files before your have crawled through each line of them. In all my recent hacking attacks the malicious code was placed in the themes header and in the root htaccess file.