Youtube has been running adsense on nearly every single page of its site for a while now. I was able to do a test campaign that ran on Youtube and was displayed at the rate of 500,000 pageviews an hour for the average price of 30 cents a cpm.
This article says youtubes pageviews are around 250 million a day. If that is even remotely accurate youtube is making a LOT of money. Lets say only 200 million pageviews a day can have ads. That gives you $300/million * 200 = $60,000/day or $1.8 million a month in revenues from adsense.
I guess everyone at youtube wants the world to think they aren’t making any money so that all the people whose content they are stealing won’t come asking for money.