The last time I checked, they weren't open to publishers, so are they now open for the general public to start serving ads? Are they selective, if so?
Publishers in the US can apply for the MSN Content Ads beta testing: http://advertising.microsoft.com/microsoft-adcenter/content-ads/signup Heres the FAQ for the Content Ads beta: http://advertising.microsoft.com/microsoft-adcenter/faqs/content-ads
I went to the listed url and to signup for PUBLISHER , YOU NEED TO BE AN EXISTING ADVERTISER , and then to be a publisher it's by invitation only.