Google introduced video overlay ads for YouTube, as this article on read/writeweb says. As a user I don’t like the idea of these ads“interrupting“ me, but it will infact be a good way of better monetizing the video experience. They’re offering it on a CPM basis for now, which seems odd to me, but I guess that might change once they know how well it is accepted?
Also, they don’t seem to be the first to launch this (by far not), as this Tecrunch article states.