Just came across an article from early March about Yelp in the NY Times: "The Review Site Yelp Draws Some Outcries of Its Own". The article is not completely negative, but it does quote a number of business owners and Yelp reviewers that feel they have been wronged by Yelp.
This is not the first article of this type that I've come across. It seems every six months I read some article about Yelp and the controversy surrounding its five million business reviews (e.g. "Online review policy has local business owners Yelping mad").
But there's no such thing as bad publicity, especially for a content web site that is free for users. Yelp's traffic has more than doubled over the last year. I'd be quite happy to have a long article in the New York Times gently chiding Nelso while touting its rapid growth and high-quality business listings.