I bet that even Barry Schwartz lost the count of these Google updates but let Matt Cutts talks instead of me:
Minor weather report: Update of goo.gl/OpIDL launching today. ~0.7% of English queries noticeably affected.
— Matt Cutts (@mattcutts) October 9, 2012
By Cutts (head of the Google webspam team), this update is on page layout algorithm improvement. Since Google is trying to improve their user experience, they are going to devalue websites with too much ads above the fold.
Google on their blog, on this topic said:
We understand that placing ads above-the-fold is quite common for many websites; these ads often perform well and help publishers monetize online content. This algorithmic change does not affect sites who place ads above-the-fold to a normal degree, but affects sites that go much further to load the top of the page with ads to an excessive degree or that make it hard to find the actual original content on the page.
So I’d recommend to you to watch out and to not place too much ads above the fold. This is considered as small update but it’s still very noticeable.