Yes, there's a pretty good sized update in progress, and has been for 12-18 hours or so.
Rankings are bouncing, particularly in the UK, like a seismograph during an earthquake. Oddly, some sites seem unaffected, some are up across most keywords, others down across most, and finally some with mixed results.
As we've seen over the past 6 weeks, Google has been making peaks and valleys in ranks, with some jumping/falling 10, 50 or 100 points, staying for a few hours/days, then returning to their former position.
Perhaps that was the prequel to this update. Only time will tell.