Often SEOs notice that a search engine may test various result sets in the search results between a search update and an other search update. It appears from one of MSNdude's most recent postings that MSN does not do that.
MSNdude in response to someone saying that at one point his site appeared, then disappeared and then reappeared soon late, said;
Despite what some might say, we don't really randomize the results with each update. :-)
Forum discussion at WebmasterWorld.