In August, Google released a developer preview of their new index, dubbed Google Caffeine. Changes in the index appeared to be largely a "Your Mileage May Vary" situation, with some noticing hardly any changes to their rankings while others noticed bigger changes.
Now, Google has closed the preview sandbox and is prepping the new index for a larger release. It will initially roll out to one data center.
I would imagine that for many, this isn't the greatest time to be experiencing changes in Google's index. The holidays are coming and the economy has been in a rough place for quite a while now. Changing up rankings in the biggest search engine could affect holiday revenues and that's a scary thing. Although, for others it will probably be a positive change as well.