manny123 - 9:45 pm on May 1, 2012 (gmt 0)
For what it's worth, I saw a small drop last week that I assumed was the penguin update. Over the weekend, traffic went up quite a bit getting my site back to January levels before all of this years' algo updates started to trend my site downward. The site has roughly 300k visitors per day and is mostly long tail, to help give perspective.
Anyway, I thought I would throw that date (April 28) out there as a day when something definitely changed with my Google traffic.