[1]: http://en.wikipedia.org/wiki/Yelp,_Inc.#Criticism_of_reviews is a good place to start digging, check their cites.
All I can find is this the dismissal of Levitt v Yelp where the Court, while "sympathetic to the ethical underpinnings of Plaintiff's argument" rules that editorial "immunity still applies regardless of whether the publisher acts in good faith", noting that "traditional editorial functions often include subjective judgments informed by political and financial considerations" [1].
[1] http://www.scribd.com/doc/70421921/Levitt-v-Yelp-Dismissal