This is the next big thing with the internet and it is going to put a hurting on Angie's list, Yelp, Tripadvisor and sites like that.
The main operation right now is reputation.com they are really not much more than a lead generation company for thousands of law firms across the country.
Doctors, dentists, contractors, hotels etc are going after people who write bad reviews about them. The majority of the time the reviewers back off on the negative stuff because they are scared by the original letter that they receive.
http://www.reputation.com/