>> This proxy hacking may be more insidious than you thought

Uhm... I appreciate your opinion, but maybe it's still not so easy.

Check this- this is what you serve for googlebot

http://web-sniffer.net/?url=http%3A%2F%2Fwww.mytrap.com&submit=Submit&http=1.1&gzip=yes&type=GET&uak=7
http://www.rexswain.com/httpview.html - type the preferred version of your domain and in the user-agent field "googlebot" (I know that's not the exact string, but in this case enough to mislead the user-agent check.

And the same for users arriving from anywhere google:
http://cli.gs/Sz58QR - are you sure that your server is not hacked???