signor_john - 3:10 pm on Jun 10, 2009 (gmt 0)

As for the reasons - this is where we can start to guess. Is it a reduced willingness to buy (probably), issues with targeting (also probable), Adsense blindness in general (likely)? We don't know. Google keeps us perfectly in the dark over this.

"Reduced willingness to buy." That sounds likely, given the constant barrage of economic bad news since September, 2008.

"Issues with targeting." Depends on the sector and the publisher, I'd guess, but most of the ads that I see on my site are extremely well-targeted (more so than they were a couple of years ago, in fact).

"Ad blindness in general." It's certainly possible that the ubiquity of AdSense ads has led to a gradual increase in AdSense ad blindness, but why would ad blindness have taken a jump in October?