PPA is certainly a great relief for advertisers. An interesting question, though, is whether such an offering is actually beneficial to the publishers? Domain-parking sites, in particular, might fail to generate profits through PPA and therefore stick with CPC instead. This advertising model might fail to scale if it is not mutually beneficial for both, advertisers and the publishers.
I think you'll see minimal impact on the landscape for publishers-- if they used CPA/aff before, they'll continue to use CPA-- Google is just another CPA source to them. Same with CPC.
The only shift I can imagine happening is that some CPC publishers will shift to CPA, as the overhead associated with evaluating the various affiliate brokers and opening accounts, etc, just got eliminated, as the publishers already have an existing relationship with Google.
