Seems like 18th December update got rolled back
To the guys that got hit: Did you have any 301s pointing at your domain?
We track several hundred sites and a handful dropped today and besides a fairly clean link profile, they all had heavily anchored/linked 301s pointing at their money site. Of course correlation doesn't mean causation, but it's something interesting we found by looking at our data. Having said that, the rest of the sites we track are doing just fine and dandy.