I covered this at Search Engine Land, Google Launches Pay Per Action Ads. Google is basically allowing AdWords advertisers to offer a flat dollar value to AdSense publishers, if the customer hits a desired goal. So instead of paying per impression or paying per click, the advertiser will only pay when a desired action is completed.
You can view more details at http://services.google.com/payperaction/. To be clear, this is a US beta only, and it is a beta for both the AdWords side (see AdWords blog) and a beta for the AdSense side (see AdSense blog). If you want to sign up as an advertiser (AdWords) go here, if you want to sign up as a publisher (AdSense) go here. Supposedly, I will be getting a beta account for both soon.
So let's hit the forums, where most of the discussion is at WebmasterWorld.
The discussion is mostly about, is this an affiliate program or not? In many ways it is but in some ways it is not. The main distinction is that you can not offer a publisher a percentage commission on a sale. You can only offer a flat dollar fee to publishers. I wouldn't be surprised if they add a percentage and commission type of structure, so that it can be a true affiliate management product, but right now it is not.
There is a ton of blog coverage on this, see Techmeme for more.