Google started to roll out panda 4.0 version algorithm update. After lot of other updates, this one is also closely watched by many seo professionals. Because this time, there might be a penguin refresh along with the regular panda update as well.
Read the twitter status of matt cuts here https://twitter.com/mattcutts/status/468891756982185985
As many of you already know, google panda updates are rolled out in order to prevent poor content sites from topping the search engine rankings. In earlier updates articles that are copied and other poor spinning article posted sites got affected by panda.