Google Pay Per Action is a new pricing model available to advertisers to specify a dollar amount they would like to offer publishers for successful conversions. Advertisers will only pay upon a completed conversion and not based on a click or impression of the ad. By expanding the Pay Per Action model to more publishers, advertisers who are testing the product can better see if the model will work.
Right now, there is a lack of data collected by advertisers for them to make a decision on the effectiveness of such a pricing model.
Postscript: Here is the official press release from Google.