According to a post on SlashDot which itself cites an article from the Wall Street Journal YouTube's management team are finding it very difficult audit the site's content for valid copyright signoff.
I don't envy them as the shear rate at which the community upload new content is staggering.
This is an interesting time in viral community informaton sharing as parties like YouTube have grown from nothing to hugely valuable & influential content repositories.
The recent licensing agreements between YouTube and copyright owners demonstrates the speed at which the medium is maturing.
Consider Napster for a moment - it grew in a similar way and went from being viewed as a threat by many record labels for copyright violation and yet it matured into a licensed content provider.