Threadwatch spotted a IT Pro article that claims some invites have gone out to publishers to test a new content ad system that works on a CPA (cost per action) model, as opposed to a CPC (cost per click) or CPM (cost per thousand) model. The article says, "The search engine has invited a number of web site owners to take part in the pilot programme that will see site owners getting extra payments if their visitors take some kind of beneficial action on the advertisers' sites - such as buying a product or requesting further information." Those invited have "greater flexibility" with the ads, as opposed to normal AdSense users. Also, CPA ads tend to "be less susceptible to click fraud."
Postscript by Jennifer Slegg: This has been in beta testing since June and it is still be actively tested by Google at this time.