The human review would certainly cause scalability problems. Amazon.com already does PPA with their book referal program. For instance, they pay their ad publishers only when their users purchase a particular book through their site.
I see this as Google getting into the affilate marketplace, and integrating the adwords buying (which many many many affiliate marketers use) and affiliate payout into one financial transaction.
Hmmn, thinking about this more, it's actually cutting out the middlemen in a vast number of affliate transactions-- those that are entirely reliant on adwords buys instead of eyeballs coming in through a particular publisher. Now you can buy affiliate clicks on Google's entire network, instead of cutting individual deals with lots of sites and dealing with the administrative overhead of managing affiliates.
Smart.