Over 70% of AdAge Top 100 marketers ran campaigns on YouTube in 2008

"Over 70% of Ad Age Top 100 marketers ran campaigns on YouTube in 2008. They're buying our home page, Promoted Videos, overlays, and in-stream ads. Many are organizing contests that encourage the uploading of user videos to their brand channels, or running advertising exclusively on popular user partner content (see Carl's Jr.). Advertisers just want control, so we're continuing to develop tools and targeting products that give advertisers more control over where their ads appear on the site."
Source: Google blog post 20th July 2009
Note - Google are very vague in the post about what sort of advertising it was, but I'd assume it was a combination of banner ads, video ads, and paid for pages, for example for competitions