The server my site is on was infected using the (pardon my obfuscation)
Y-E-S E-x-p-l-o-i-t S-y-s-t-e-m
It is a "control kit".
I found out some info about it from http://cassandrasecurity.com/?p=282
How can I remove this control kit and secure the system? Trojan scans reveal an infected GD library.