Search Engine Marketing (SEM) through the use of Pay Per Click advertisements within search results has been made famous by Google - although the process was invented and originally patented by Goto.com, which became Overture and was acquired by Yahoo Search Marketing. MSN recently introduced its own portal for paid search advertising, known as MSN adCenter. They are phasing out of their relationship with Yahoo Search Marketing, which has traditionally provided the sponsored listings within MSN Search results.
Since the launch of the product in the USA, various members of the SEM community have had a chance to participate in the pilot, including SEW Moderator Aussiewebmaster, a very talented in house search marketer based in New York. On the 11th, he posted some interesting changes coming to the system which have been predicated by user feedback. Having been involved in the pilot myself, I can vouch that MSN has been very proactive about communicating with its advertisers. However, there have been problems associated with this system, as could be expected for any advertising platform of this magnitude.
With MSN's recent increase in exposure discussed in March, advertisers will be anxious to see improvements in the system.
To learn more about the changes, see the thread at Search Engine Watch. This thread is in the SEW adCenter area. Other forums with areas dedicated to discussing MSN adCenter can be found at Webmasterworld, and Digital Point Forums. Various threads can also be found at High Rankings Forums and Cre8asite, among others.