Pjman - 11:00 pm on Feb 11, 2012 (gmt 0)
I switched from all Adsense for the longest time (4 + years) back in June to a small (my niche focused) ad company. That is right around the time my Google referral traffic dropped a constant 40%. Only a little recovery since.
Only problem was that this was also right around the time of Panda 2.2. Mine traffic dropped June 18. Panda 2.2 hit on June 16. I'm not sure there was an official date on Panda 2.2 anywhere.
I'm in the process of switching my ads over to google ad manager to serve the same network ads + adsense. Hopefully that will help too.