Vario Creative Blog has a nice piece about pay-per-action advertising, which Google is now doing in a beta.
The idea is incredibly appealing for advertisers: instead of paying for an impression, or a click, you pay for a conversion. Since that's really the point of many programs (especially search marketing programs), that's a great thing. (And, as an advertiser, I'm excited about it.)
But... there's a problem here. This requires media to take responsibility for something beyond their control: the ability of your web site to turn visitors into customers.
It will be interesting to see how Google, and others, manage this. It seems to me that there are going to have to be some quality standards for advertisers who want this model - if you can't demonstrate an ability to get conversions, no medium is going to want to put their revenue at risk on you.
Stay turned to see how this turns out... it should be interesting.