In a provocative article in today's iMedia Connection newsletter Carat's Ron Belanger (with whom I shared a long table at the iMedia Summit in January) suggests that some paid-search networks adopt a cost-per-acquistion/cost-per-action model to combat click fraud.
Regardless of whether the named engines/networks adopt his advice, I believe we'll see more and more CPA models popping up over time. Part of this will be driven by advertiser demand and part of it by the publisher/network's efforts to differentiate from larger competitors (per Belanger's argument).
For example, imagine a new shopping engine. How would it compete in an extremely crowded field? One way would be to offer a cost-per-coversion model as a differentiator. As more and more CPA (or equivalent) models pop up, PPC/CPC models will face pressure.
Ultimately, however, CPM, PPC/CPC and CPA will all live side by side, because they address different advertiser objectives.
Webpronews (per Battelle's post) reporting on a recent click fraud survey finding near 30% click fraud levels, which I don't entirely believe.