Murk - 5:37 pm on Feb 25, 2011 (gmt 0)
One of my sites is over 15 years old and page one for quite a few keywords - 90% of its traffic is from search engines to legacy evergreen content. We monitor traffic in real time and have watched an immediate 30% drop beginning yesterday. The reduction is consistent in our hour by hour stats.
Per comments above, we use a dozen ad networks including Adsense, which accounts for 5-10% of total income for that site.
Many of our keywords are still coming up on first pages so I'm not sure where the drop is occurring. I have noticed other search engines are now appearing more often in our SERPs logs so Google is definitely cutting out some of our content that others still find. I'm hoping Google tweaks its algorithm again after this test - they've done that before.