Google just entered the PPA (ie CPA) (ie lead generation) market. Not that it's rocket science or much of a leap, but I had predicted this a long while ago. I don't feel like digging for the post. Ok. It wasn't that hard to find.
They just took the "smart-requirement" out of using their system, as well as the "you need to invest $500/month and wait 2 months to figure out whether it makes sense to use their ad network" out of the equation. I imagine there's still incentive for the advertiser to optimize creative so that it gets clicks and conversions, and gets publishers to sign up. But, there's no risk now. Forget adsense arbitrage. Just set what you're willing to pay for a lead or a sale and g-g-gooo. No need for the advertiser to manage risk.
I got excited when Turn launched. But, they haven't gotten much traction from the publisher side. Without inventory or affiliates, CPA doesn't work. Google has a HUGE network of affiliates, not to mention their own inventory. So, this is finally an automated CPA system with access to inventory. Or it least it could be.