Seem Google will chase SE spammers with new remote workers
Danny Sullivan, a man generally held to be in the know, provides some insight into the reasons behind the major SE's hand tweaking certain result sets as well as a practical example of Yahoo! doing this (alledgedly :-)
It doesn't take a search genius to realize that hand tweaking goes on quite alot, particularly on the kind of queries that were they to be poor, would attract negative media attention. Greg Boser made a good post about this detailed here.
Although TW is hardly a search engine, i often hand tweak the recent posts list by choosing which posts to reply to in which order :) It's a natural enough thing to do when the normal algorithms or mechanics of a system fall short of ideal.
It's a great technical post by danny, go check it out.