There are many signals in the webmaster forums that show that there was a possible significant Google ranking shuffle yesterday morning and throughout yesterday. I suspect as webmasters wake up and look at their analytics it will help confirm or deny this.
Now, this might be Google testing a new algorithm, or testing an update to an algorithm. Or it might be a large number of individual sites received manual penalties. It can also be a bug with Google or even nothing at all. Time will tell.
Did you notice major ranking changes yesterday in to today?