Jonny6 - 8:20 pm on Jan 10, 2011 (gmt 0)

I dont think its any form of traffic throttling. The dates you provided are the most significant factor.

"The theory is that sites we have that are in one cluster of google's world get to be guinea pigs for the algorithm changes, with wild and unpredictable results"

"Its almost like a completely different dataset was being used for serps"

Agree with both suggestions, Google is using an old dataset from early December with some data coming from a new dataset. The new index is not being introduced gradually, its sporadic and evident of still being under quality testing. Once this testing is complete, it will be implemented permanently, just like mayday.