This is an interesting development which can be interpreted from two major perspectives;
1) Google is bringing out their version of an affiliate program, so long Tradedoubler, Linkshare & Comission Junction
2) Google continues to refine their model to find out the best way to extra maximum revenue
I’m in the second camp, Google has experimented with CPM (and now actually offers it as an option on the content/AdSense network), mobile advertising (though granted not as well as Yahoo!) with video pre-roll ads and they acquired Adscape last week to find out about in-game advertising. Google’s revenue driver is advertising, and they need to be aware of what is going on in the market and continue to test different formats to find out what delivers them their best ROI.
I don’t think the impact will be felt by people who are already running with a decent PPC budget already, they’ll be working back to some CPA target anyway, so all the CPA portion of the deal will do is allow Google to manage it as opposed to doing it inhouse.