YouTube blog: "Today [Oct.7], we're taking our first steps to providing YouTube users with this kind of instant gratification, by adding "click-to-buy" links to the watch pages of thousands of YouTube partner videos. Click-to-buy links are non-obtrusive retail links, placed on the watch page beneath the video with the other community features."
Apparently, not all of the wrinkles have been ironed out yet. The Sims 2 trailer above has an Amazon link to Spore (both games are by EA that is among YouTube's new partners). YouTube's aff id with Amazon is you09f-20 (probably one of many).
Incidentally, a similar affiliate model could work for Facebook, if the virtual gifts stats are any indication. See how Birthday Calendar app inserts a Send Flowers affiliate link to 1-800-Flowers.