Fyi, Dreamhost is responsible for the infastructure that you host your site on, they are not responsible for the software you install there. Also call back support is available for an extra charge.
That said check your .htaccess file. Download a copy and scroll completely thru it, there are two other threads i'll try to find shortly that discuss this exploit. In at least one of the other cases you need to scroll down past blank space and possible scroll right to find lines that have been indented to hide the new entries.
If you have a backup copy of your .htaccess file check it to make sure it's not corrupt and replace the copy on the server.[hr]
read thru this thread: http://discussion.dreamhost.com/thread-130973.html around post 10 it shifts to wordpress as well
the exploit is most likely caused by an insecure theme or plugin that you added.