This is one search feature I did not know MSN had until I was pinged by a Syndk8 Forum Member about this thread which has the details (I know the forum is locked down, sorry). Basically, if you do a search in this format at msn search; ip:64.236.116.57 you get a listing of all the sites hosted on that particular server. The information on this search feature is written up at MSN's Web Search Help page.

Locked down forum discussion at Syndk8 Forums.