Fred Wilson's got a great take today on post-roll advertising on YouTube...worth quoting at length:
The second link in the post-roll frame should be a bidded marketplace like the right column on the search engines, but it should be only for videos on YouTube. Say you are a marketer, like Nike. You make a video, doesn’t matter how long or short it is, about your new Nike sneaker that works with the your toaster so breakfast is ready when your run is over. You post that video to YouTube in your brand channel (which already exists on YouTube) and you buy a bunch of tags (user generated keywords) that you want that ad to be linked to from the back frame. You pay on a CPC basis, only for the clicks that come from that link and lead to plays of your video. If your video is great and the audience loves it, passes it around, etc, you can probably stop buying traffic to it and rely on the organic linking and viral nature of the service to keep the video playing.
Two additional thoughts on this. First, YouTube is in a position to do this today because of their scale and reach. They dominate online video, and the fact that they make it easy for that video to be distributed around the web has dramatically contributed to that dominance. I know this is stating the obvious, but it's not like every one of the video sharing sites has the reach to pull something like this off.
Second, for brave brands there's no reason why those post-roll ads need to link to their own produced content; how great would it be if Nike decided to link to content that's created by their customers, and somehow worked them into the revenue stream?