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It's surprising, given the upward trend in the fall of searches. I think the latest round of changes Yahoo has made, is having a pretty profound effect on referrals.
WRT to other SERPs, Chris_R brought up in another thread the idea of the amount of choices available to the user on a search results page. As this goes up, traffic to any one element probably goes down. The addition of GoTo results has had this effect.
Finally, as more and more sites enter the directory the chance of finding any one getting a click goes down. Look$mart will probably experience this to the largest degree since they will accept just about anything, but as YAHOO! grows, the referrals per listing should continue to decrease unless the number of searches increases accordingly.