This update was unannounced by Yahoo and has hit webmasters by surprise. The last update was on July 17th but that update was not as significant as this update appears to be. Here are some of the responses and reactions from webmasters.
Our site has lost about 30% of traffic
Many of my keywords that ranked #1-2 for years have dropped 10 spots or so.
Their last change is complete disaster. New SERPs prefer totally irrelevant, unimportant or simply just not the best sites. No wonder why they need help from Google or Microsoft to survive
Many feel that Yahoo will reverse these changes, at least at some level. But the update began two days ago and the changes are still noticeable.
Update: Yahoo posted tonight that this is an official update.