On the whole is seems that the MSN search engine is indeed placing IIS hosted sites higher in the results more frequently than other webservers. Frequently the MSN search is placing more IIS servers in the important top 10 results than Google even where result sets from a query have actually returned fewer IIS servers overall on MSN.
Looking at the coverage graphs, most search phrases return a more even spread of IIS servers thoughout the results sets from the MSN searchs.
So what's going on?
I have no idea, I doubt it's all a big conspiracy... but some possible explanations spring to mind:
Perhaps the MSN search has simply been coded by developers used to talking to IIS machines and so it just does that job better?
Perhaps the MSN spider is taking advantage of some specific IIS features to provide enhanced indexing?
Interesting enough, I have never seen a thread at an SEO forum with this sort of speculation.
Update: Looks like I was wrong, DigitalPoint had a thread on this topic for a while now.