Consider this in the “for what it’s worth” category but for a while now it has appeared that Google et al have provided better rankings to sites that are hosted on dedicated servers. Now, there is some evidence that the new Microsoft engine provides better rankings to sites hosted on IIS. As the article says this could be a result of the people familiar with IIS developing the MSN search spiders and doing a better job of indexing IIS sites or it could be that they are taking advantage of specific features in IIS to provide enhanced indexing. Or, it could be a coincidence:-)
In any event, sounds like hosting your site on an IIS dedicated server will allow you to get the best results and hedge your bets with everyone.