Just got this email from my AdSense rep:
We're rolling out a change to our text ad formats to help reduce accidental clicks and increase value for advertisers. Previously, users could click on the background and full text of an ad, but now users can click only on the title and URL of a text ad. This new format will match the changes we've implemented on Google.com to help decrease the number of accidental clicks and increase the number of ad conversions.
A reduction in accidental clicks will keep users on your pages, interacting with your content until they choose to click on an ad. This change will enhance the users' overall experience with your websites and improve advertiser campaign value, but it's likely that your click-through rate will decrease.
I want to assure you that we approach these changes carefully, with a view to the impact on all parties involved. Although we recognize these changes may result in a RPM decrease for some publishers in the short term, we expect these improvements will lead to increased advertiser spend in the long term.
By investing in the long-run health of the relationship between AdSense publishers, AdWords advertisers and your sites' visitors, we're ensuring your continued success as an AdSense partner.