I m sad for that . I m a security specialist and I know this Lamer Probably problem about your hosting services(got a root server ) or wp version. milw0rm.com Wordpress bugs.
Thank for the advise. My host already step up the security issues. BTW it is simply unethical to do such a thing - for whatever reason.
You better find the exploit he was using to breach-in. Go through your logs, change your passwords and all of these.