I've been wondering when this would happen: Someone has filed a class action lawsuit against Yelp for unfair business practices:

The plaintiff in the suit, a veterinary hospital in Long Beach, CA, is said to have requested that Yelp remove a negative review from the website, which was allegedly refused by the San Francisco startup, after which its sales representatives repeatedly contacted the hospital demanding payments of roughly $300 per month in exchange for hiding or deleting the review.

This will be an interesting case to watch.