With all the dollars at play, I reckon there could be unforeseen ways to game the system. Maybe some "Enron" type will scrutinize every wikipedia bureaucratic procedure and term and condition of how the ad sales works and run some arbitarge keyword games that would distort the content in an ultimately un-usefull-to-the-user but profitable-for-the-gamer way.

I also worry about the increased tracking on politically sensitive pages or controversial topics.