A thread on WebMaster World reports that some advertisers are receiving emails from Google to test a new addition to the Adwords service called Site Target.

Site Target first got noticed in April 2005 - It is essentially a whitelist of websites advertisers want their adverts to appear on. Unfortunately it doesn't allow you to block particular website, and also, currently it only works for image ads (I imagine this will change).

"Since site-targeted campaigns are tied to specific URLs, they won't appear on Google.com search results pages, on the Google search network, or on associated products such as Gmail. Keyword-targeted ads will continue to appear in all those locations."