There was a famous sting operation in Florida, "Operation Flea Collar."  You can do a Lexis-Nexis search (or similar type search) and read all about it.

No, say it isn't so?  Police officers would actually pose as prostitutes?  They would actually try and lock somebody up for paying to get laid?  Man, what a crazy world.  I would like to say hello to all the cops reading this.  Time to get the donuts!!

St. Louis just organized a major operation using internet ads. They posted ads and the phones rang to their vice squad who had ladies answer the phones, then set up an outcall to peoples homes/hotels.  

I believe they also did a reverse sting from the internet ads where they had guy cops call ads and nab the ladies as well.