I'm not totally sure what the difference is between RPO and hiring a headhunter. Perhaps there is more technology invloved, but a headhunter does many of the same functions. basically we are talking about replacing the recruiting function in your organization. Now, the RPO will write your job description, strategize and interview (prescreen) candidates for you, and do all candidate communications. Here are a few links
HR Outsourcing Awards: Accolo wins the prize for RPO of the Year
Accolo: What is RPO and what do they do?
The Lucky 13: overview of 13 companies who are providing end to end RPO