Our research suggests that the people actually ARE clicking less. It began in October (after the meltdown of Lehmann), and it still has not recovered. This statement is based on our independent click-tracker that for a long time now has captured the clicks on Adsense. In October, the number of clicks decreased significantly, and this move was reflected both in the official Adsense stats and the independent click-tracker stats.
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.