My money is on a big Panda update. I'm seeing drastic changes everywhere I look.
Something is definitely brewing. We had huge gains overnight in kw positions. We have more pages in the top 10 than... well, I'd have to pull historical data it's been that long. Lost a portion of our backlinks (5K about 5% of our total) since yesterday. Our traffic is definitely above average so far this morning, but not anything like pre-panda levels. And the most interesting is that googlebot is going crazy on our site. It has already spidered more pages than it does in a normal day, and it's only 9:30am here.
We've lost over 60% of our traffic since Panda 1.0. What I wouldn't give for just even a small recovery at this point.