The adCenter Blog gives 'open invitation to register for US Content Ads.'
The adCenter advertisers who want to place their ads in Microsoft's contextual network can register with the Pilot Sign-Up form . According to the blog: "Participating in the pilot will not only offer you access to our high-quality MSN inventory, but it will also allow you to see a technology preview of a few new UI features. Content Ads offers the same flexibility as Search, as you can set separate keyword bids for Content, and still use advanced targeting settings."
Learn more about content ads at Content Ads Overview .