The ongoing WebmasterWorld thread has a huge spike in chatter over the past 48 hours with speculation around an update.
In the past 48 hours or so there have been over 100 new posts in the thread, all around webmasters and SEO asking if there was some sort of update.
There was a Google attack on a link network recently, which may be related. Normally when Google breaks down a link network take some time, so sites would see the results (i.e. a downgrade in ranking) over the next few days.
But this may not be related to the link network crack down.
It seems many are suspecting a Panda update. We are indeed due a Panda update, it's been almost 7 weeks and we average a Panda update every 4 weeks or so.
Did you see signs of a Panda update?
Forum discussion at WebmasterWorld.