I just finished reading Return Path's Sender Score Certified: Standards for Reputation-Based Email Accreditation; July 2006 white paper by George Bilbrey, General Manager of Delivery Assurance.  From what I can tell by reading this five page white paper, Return Path's new Sender Score Certified is a program to replace their not so popular Bonded Sender Program. Instead of putting the cost on bulk email senders, ISPs will pay to use Return Path's scoring system to determine if a bulk email sender has a decent enough reputation to send email to their customers.    

Return Path's scoring system seems sophisticated and in theory could work however I have yet to see a white list that prevents spam.  Spammers are usually a few steps ahead of the industry and I'm sure they are studying Return Path's white paper too.