In the Google earnings call, that we reported earlier, Google's Senior Vice President, Johnathan Rosenberg, said on the earnings call:
AdSense revenue was weaker, though AdSense for content was strong. When it comes to AdSense for Search, we did a lot of arbitrage cleanup in Q3 and Q4.
This quote comes from a WebmasterWorld thread, and discusses the significance of such a statement.
In Q4 2007 Google earned 1.31 billion with AdSense (and that network), but in Q4 2008 Google earned 1.29 billion. Even in the third quarter, Google earned 1.33 billion. So there did seem to be a downturn recently in that area.
Some are using this statement to reduce their estimated tax payments in 2009.
Forum discussion at WebmasterWorld.