We've been discussing this in the Google "throttling" topic, but no definite answers. My sales are WAY off from this time last year and it all started with the Google's Mayday update on May 18th 2010.
We had a semi recovery in June, July and August, but then around September 1st everything just shut down again. Our traffic is the same or better than last year when we were doing 10 to 20 sales per day. Now it's 4 to 6 sales per day if we are lucky.
There have been no changes to our site and I've even included coupons in an attempt to promote sales. The bottom line is that our traffic, that has been great for the last 10 years is now either very un-targeted or from foreign countries. Our logs show that our traffic levels have not dropped at all.
I have notice a large increase in sales from places like Australia, and South Africa. Weird! I feel for all those who are suffering the same drop in sales, you're not alone.
It's my opinion that the recent introduction of Google Instant has everything messed up again.