Optimize for google and you will rank well in the 3 major search engines. If you rank well in Google, chances are that you will rank well in Yahoo and MSN as well. However. If you rank well in MSN, there is no guarantee that you'll do the same in Google.
I have noticed a lot of the people that are older and do not have a lot of computer experience use MSN as a search engine because it is their default. Because of this I would like to suggest that showing up well in MSN will help bring an older demographic to your site. Do any of you have feelings on this? Am I up in the night on this one?