Both Mozcast and SERPS.com reported significant fluctuations on Wednesday, December 5th.

I checked WebmasterWorld the following morning but it was temporarily offline and due to that, the discussion of a possible update was very low to none. So my normal gauge to see if there was an update was broken.

That being said, there is some chatter in the WebmasterWorld thread that there was an update late last week that webmasters began to notice on Thursday, Friday and Saturday. One webmaster said:

Did you notice an update late last week? Should I ping Google asking them about this one?

Forum discussion at WebmasterWorld.