BillyS - 1:44 am on May 11, 2010 (gmt 0)
The most compelling evidence I've seen comes from pontifex. I've stared at our data for years now and the change patterns are pretty clear.
Unfortunately, as was pointed out by Reno I've got some seasonality at play too. Our traffic normally drops after the tax season is over in the US, and continues to degrade through the summer months.
WMT is a great resource, especially slicing the information by date (April versus May). I do see a lot of shifts, but not nearly as bad as someone like pontifex is likely seeing.