This is what shows up at the bottom of the hacked sites:
How are the sites getting compromised?
On the sites we’ve analyzed, they were hacked through the timthumb.php vulnerability that was published a few days ago. The attackers are also creating a bunch of backdoors to maintain their access to the hacked sites.
If you are using the timthumb.php scripts, remove or update it now!.
Keeping yourself secure
This is not a vulnerability in WordPress, it is a vulnerability found in various WordPress themes that include TimThumb! You have to make sure that you are using an updated theme, and from a legitimate source. Otherwise your theme may contain this vulnerability, or others (even backdoors), that may not be given the proper attention by their theme authors.
If you’re not sure, you can do a free scan of your site using Sucuri SiteCheck