It looks like webmasters recently felt the affects of another Google Panda Update tweak. Since what's come to be known as version 2.5, Google has been making tweaks to it as promised, and these tweaks continue to help some sites and hurt others.
Search Engine Roundtable points to some webmaster discussion, indicating that a tweak may have rolled out on October 20. There are indeed plenty of complaints in a WebmasterWorld thread here. Most are calling it a minor update, but even the minor updates can be major for some sites.
User Bewenched wrote, for example, "It may have been a 'minor' update, but it slammed our ecommerce site yet again... down 30%"
We've reached out to Google for confirmation that this was indeed Panda. As you may know, they make hundreds of algorithm changes a year. We'll update when we receive a response.
Update: Google tells WebProNews there was no Panda tweak on October 20.
If you feel like you've been unjustifiably hit by the Google Panda Update, Google recommends that you let them know in this forum thread. For other resources, that may help you, we recently created this Delicious stack of Panda resources (which by the way is still a work in progress. Feel free to share other resources you think would be helpful, and we'll consider adding them).