Older installations of WordPress have recently experienced a new wave of attacks as they have been increasingly targeted by hackers. These installations are highly susceptible to a variety of attacks. What to do, then, when your installation has been comprimised? Here's a good list from WordPress of the steps to take when your WordPress installation has suffered a successful attack.

http://codex.wordpress.org/FAQ_My_site_was_hacked

The HP Web Security Research Group's own Matt Wood recently wrote some excellent advice for a hacked site, as well. Each of these lists will help you secure your WordPress installations.

http://www.communities.hp.com/securitysoftware/blogs/spilabs/archive/2009/08/19/advice-for-a-hacked-site.aspx