"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?