How many PPC advertisers think they’re targeting search, when in fact a high percentage of their ad displays and click-throughs come from expired domains (or for a couple hilarious weeks, mistyped domains), pop-ups and pop-unders, adware/scumware, etc.? Do the PPC providers openly disclose this side of their distribution network, or do they sort of admit to “having a partnership” when they get cornered?

Good point. Though I wonder if the ROI for advertisers is significantly different than with actual search engine results, vs. PPC parked domains.