YouTube is experimenting with inline ads, showing a text ad at the bottom of its player as a video is playing. If a user clicks on the text, a video ad expands and appears layered on top of the player. The ads are also clickable from a static post-roll. Examples are shown below in screenshots from a My Chemical Romance music video.
Update: Embedding the video here to see if the ads come through when the video is on other sites. Update again: It appears they don’t (though let us know if you see them). Looks like the little yellow (rather than red) time bar indicates an ad.