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 72.14.194.17, which is part of Google's 72.14.192.0/18 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?