A revolution has taken place in the world of seo. Google is doing its best to combat spam sites and it does it via the Penguin update. The mechanism of this update is quite simple. It marks site with low quality content and low quality backlinks and punishes them. It can happen cpnsequently that a site that reached high place on google will suddenly drop siginicantly or even might be unindexed.
True, some good sites may have suffered as a result of the update, but all in all it is another attempt made by Google to make sure the Internet users will enjoy real materials and not materials that were created for spam and SEO.
If you are an owner of a website, just make sure your site contains good material and lots of it and then you will have nothing to be afraiod of
For further reading try this post on Penguin Update. This posts gives some further information