In the WebmasterWorld thread, in post number 3977195, senior member, BillyS, said:
We started to see some pretty big changes this morning.This appears to be a return to our pre-summer positions.
People first started noticing this possible update over the weekend, so there have not been too many posts on this yet. I guess we will see how large this update is as people get into the office on Monday and begin posting their findings in this thread.
The last update we reported on was a small one in July 2009.
Update: Matt Cutts of Google commented below saying he was not aware of any update taking place now.