Matt Cutts posted a video on YouTube that confirms Google's recent Mayday update and its effect on search results. It looks like Mayday addresses a change in the SERPs that affects mostly long tail searches.
He doesn't really go into much detail, but it could explain the recent drop in traffic that many sites are seeing. Although Google calls the change a winner, it doesn't appear that many people at Webmaster World would agree. Google MAYDAY Update - SERP Changes May 2010