There is a good explanation from SEO Fast Start about Google proxy hacking and how to defend against it.
I was discussing possible reasons why one of three sites were not showing up in results with a friend of mine for his celebrity fan web site. He discovered this article above about Google proxy hacking that could trigger a false positive in Google’s “duplicate content” check.
There are some techniques suggested in defending against those proxy sites whether they are intentionally (or un-intentionally) kicking your site off the SERPs.