http://www.beet.tv/2009/11/youtubes-move-to-1080p-format-is-good-news-for-madison-avenue.html
SAN FRANCISCO -- The much blogged about introduction of a higher quality playback experience at YouTube, using a 1080p shoebox sized player, was done in part to accommodate the emergence of pre-roll video advertising on the giant site. In a related announcement this week, YouTube said it would start an experiment with pre-roll ads, providing users with the option not to see the ads.
Andy Plesser, Executive Producer