I looked at our clients last night with an eye to this discussion.
A site with 1.2m monthly unique visitors and 400,000 keywords: there is no change in the traffic pattern. I ran the graph back to six months. No changes, outside of normal fluctuation.
The site is a major university (with .edu) in the USA. I assume Google gives them a high ranking value.
An ex-client called me last night; they've been on Google for 7+ years and always in the first 2-3 positions for their main keywords. They fell off the first page. Sales are down 90% (they do $8m/yr). He's increasing his PPC to make up for lost traffic and hoping that his ranking recovers.