Shifting PageRanks have been reported for the past couple of days. Now, the relevant blogs and forums are rife with reports of Google's PageRank penalties in actions from February 15, 2008.
The penalty is another PageRank update for the sites who sell links as is being talked about. The threads from DigitalPoint Forums and WebmasterWorld amongst others, who report of downgrades to their PageRank scores.
Apparently, the penalty is supposed to be a PageRank (visible ) penalty to prevent those that sell and buy links for PageRank. Perhaps, Google has found it to be an effective tool to deal with such activities in the past and hence it is frequently made use of. Recently, the Google directory was also updated, see: Google Directory Update.