This Blue Lithium study claims that ads next to what they call UGC (I'm not a fan of that term, I prefer Conversational Media) do better. From the release:

On average, user-generated content (UGC) sites provide a lower cost per conversion than non-UGC sites. That's the surprising finding from a new study published today by BlueLithium Labs, the research division of ad network BlueLithium.

Questions about the brand risk of running ads within or adjacent to user-generated content have been debated extensively in the advertising trade press. However the issue of whether user-generated content Web sites perform as well as editorial Web sites for marketers has not been explored. BlueLithium had, in fact, received queries from its own marketing clients regarding the price performance of UGC vs. non-UGC Web sites, so to help answer that question for its clients, the company undertook a thorough, unbiased, nine-month quantitative study.