Google have announced a new system that let's advertisers only pay for ads when a consumer makes an action as a result. The site says:
Increase your advertising reach while paying only for actions that you define. First, you'll create an ad and define the action that you want a user to perform when they visit your site, such as signing up for your newsletter or purchasing a product. Then you'll set the amount that you're willing to pay when this action is completed. Finally, you'll install conversion tracking code on your website so that we can verify when an action has been completed.