Given that it costs less than $20 million to run Wikipedia (cite: http://upload.wikimedia.org/wikipedia/foundation/a/ac/FINAL_... ), that seems completely attainable without needing to change the nature of Wikipedia by introducing advertising. A lone rich donor could cover the budget for an entire year, and many people who could afford that wouldn't even be mentioned in Wikipedia, so there would be little opportunity…
Anyway, they had the same issues about ads and editorial independence, yet somehow they seemed to be able to continue to be trusted despite having ads. And get this, even though they had ads, you still had to pay for them, too! Those were some strange days...