I've been noticing a lower CPC this month and been wondering why? Google +1 Buttons went Live on Display Ads October 1st. It turns out that the Interest-based Ads option has been enabled as well.
I disabled the Interest-based Ads option last night. Woolala! Now, getting a better CPC. So, something to look at. In your adsense account under the "My Account" tab are two options to enable or disable Interest-based Ads.