Sample here which works if you are using IE as your browser.
The ads appear about 15 seconds after a user begins watching a clip as an overlay. Users can close the ad, but the ads will disappear themselves after about 10 seconds. Rates are suppose to be $20 for 1000 impressions.
Perspective: Google has been testing these ads for a while, and it will be interesting to see how quickly marketeers move into this medium. I expect caution at first, until confidence is established that the right ads can be run against the right content. This initiative may also encourage more content deals between Youtube and major content distributors seeking to monetise their video content.
Update: looks like Ten (australia tv) may be about to sign a deal with Youtube, reports B&T.