Google have introduuced testing of a new part of adsense.
Publishers can now (when it goes live) get paid for an "action" instead of for clicks or impressions.
It looks like an "action" is defined as either a sale, Or a lead.
This differers from regular adsense in that it won't be contextual. Publishers will be able to pick what adverts they want to display in thier CPA campaign.
At the moment you can only get into this program via invitation.