Google's soft underbelly - there's other people out there now with large enough volumes of traffic to justify taking them on in the ad marketplace. It's worth remembering that Google isn't search....Google is ads. If they're losing ad revenue, or there are other ad alternatives, all of a sudden we may be back to the old days where Google cares about webmasters again.
Every Firefox download comes with Google as the default search engine turned on - how many people change it?
All WP has to do is make this widget turned on by default in every WP download with a link to 'register to receive your portion of the proceeds', and they will have an instant huge available marketplace - and I bet a sizeable portion of people woouldn't even register for a cut of the ads.
Something as simple as that could easily make enough of a dent in Adsense to be noticeable to Google.