There is a thread at WebmasterWorld named Yahoo Update? where members have noticed major shifts in Yahoo's search results pages for competitive keywords. GetBot in message number four says that he has been watching Yahoo! Search and noticed that Yahoo! has been serving up basically two sets of search results. He feels that Yahoo has been testing out a new algorithm that assigns more weight to those pages that have been listed in the Yahoo! Directory. His reasoning for this is that he feels that Yahoo!'s only competitive advantage is its directory.
An other point to note is something we discussed a couple days ago in the entry named Different Results in Yahoo! Search - Cluster Topic where members at Search Engine Watch Forums were noticing different results based on the case sensitivity of the keyword phrase. Tim Mayer, directory of product management search technology, answered this anomaly by saying "that the document was not being returned in one cluster and it was being returned in another" but it absolutely had nothing to do with case sensitivity. Interesting to note these two threads with Tim's response - might be some correlation.