Interesting story from Australia's The Age builds on the question I was asking earlier about social media and ads. The online game Project Entropia is letting players create ads for themselves within the game.
I haven't played Second Life, the very very popular game/community, so I don't know if this is a common thing. The Age reports that Project Entropia sells virtual billboards where players promote fashion shows and hunting competitions that are taking place within the game. (Via Adrants)