It looks like MSN care about how your website built and how it looks like (tables , TRs , TDs , etc..) more than Google & Yahoo ..
One of my websites was receiving fair traffic from Google then Yahoo and barely MSN (i do a good SEO for my websites and always update) , then i redesigned this website but with more attention to how my tables and TDs looks like.. i make it more like separate tables ( Like MSN & Yahoo ) , just 2 days after this update and MSN become my top SE referrer .. i donno excatly if this is the reason for sudden jump in MSN traffic or there is another reason .. but may be it is