I'll quote Esmi, who has been posting the best list of links in such occasions, to the point I ended up copying his list of links in my notes.txt resources file ^^
I'm sorry but you have been hacked. You need to start working your way through these resources:
http://codex.wordpress.org/FAQ_My_site_was_hacked
http://wordpress.org/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/
Anything less will probably result in the hacker walking straight back into your site again.
Additional Resources:
Hardening WordPress : http://codex.wordpress.org/Hardening_WordPress
http://sitecheck.sucuri.net/scanner/
http://www.unmaskparasites.com/
http://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html