Google has admitted in recent history that their own user-generated content sites, like Google Code and Google Reader, are a potential avenue for spammers and malicious users to host or at least refer to nasty content on the web. Most recently there has been a Facebook worm
attempting to spread using these exact tactics.
The worm will post invites to people's Facebook accounts, providing a link to a video supposedly hosted on a Google page. The video, of course, is nothing more than a link to the aforementioned worm. Google itself is not host to the worm, only the page that links to it, but it's easy to see why people would by default trust a page hosted by Google. The worm ultimately tries to squeeze money out of people with a fake virus cure, as many worms these days do.
Google obviously wouldn't let content like that stand once they discovered it, but they've become so big these days that it is surprising we aren't seeing even more of these. As always, be careful when you browse.