By definition it won't affect PPC spenders at all unless someone clicks on the ad. You can only buy CPM ads on the AdSense network.
It has the potential to drastically alter search, which thus would drastically alter PPC. The likeliest negative outcome is the gradual disappearance of any traffic driven from long-tail search keywords.
It won't affect PPC (pay per click) spenders. It will only matter in terms of CPM (cost per impression).