I am seeing a significant uptick in complaints in the forums about downgrades and delistings of rankings in the past 12 hours or so. Most of them coming early this morning in the past hour or so and more and more reports coming in via the Google Webmaster Help forums.
The ongoing WebmasterWorld thread has an increase in activity from webmasters noticing fluctuations in Google. A senior member said, "having the biggest traffic morning in months. Since the smaller data set seems to be good for me, I wonder what that will bring....good or bad?" This is followed by a bunch of other complaints about increases and decreases in rankings.
What do you think?
Forum discussion at Google Webmaster Help.
Update: Later today, Google has announced this update on the Google Search Blog.
Update: This is now named the Penguin Update.