It might have been this one:
(which wasn't about AdSense but banner ads in general - guess what, CTR is down for EVERYONE in 2012)
But it's possible it was something else; I know I read it within the past week, but I read a ton of blogs and sites every day.
If you google stuff like ad blindness and banner ad blindness, and use the advanced options to limit your search to the past month or past few months, you'll get lots of items. We're not the only ones talking about this by a long shot.