Softness in Google’s paid click metrics is a result of Google’s own quality initiatives that result in a reduction in the number of paid listings and, therefore, the opportunity for paid clicks to occur... The reduction in the number of paid listings existed progressively throughout 2007 and was offset by improved revenue per click... There is no evidence of a slowdown in consumers clicking on paid search ads [for the other engines]...
-- ComScore, 2/29/08
Jibes with our experience.
Hattip: Andrew Goodman @ Traffick