It looks like some folks are hosting proxies on Google's AppEngine.
I noticed http://proxysun5.appspot.com/ when I caught it doing naughty things to one of my web sites. Unfortunately, I cannot easily IP block it because it's IP is within Google's IP address space. It attacked from 188.8.131.52, which is part of Google's 184.108.40.206/18 range.
I found a good writeup on Labnol about how to do this, Setup your own Proxy Server in 5 Minutes for Free.
What do you think?