Google has confirmed in a WebmasterWorld thread that they are testing a new feature that will give publishers the ability to specify which sites are allowed to display AdSense ads with their publisher IDs.
The "Allowed Sites" option a new feature that is being tested on a limited number of accounts. The allowed sites section shows you a list of "unauthorized sites that have displayed ads using your AdSense publisher ID within the last week."
It then gives you the option to "Allow any site to show ads for my account" or "Only allow certain sites to show ads for my account."
I thought this might have been the site authentication feature that AdSense has been testing for a year plus now. But I was wrong. AdSenseAdvisor said:
We're starting to test a new feature that would let publishers choose between allowing any site to display ads with their pub-ID (no change from the current system), and authorizing only a specific list of sites to display their ads. This is separate from the existing Site Authentication feature.
The team thinks this'll help address a number of issues we've heard feedback on, and we hope to release it to all publishers if the testing goes smoothly.
Why is this exciting for AdSense publishers? Well, right now, technically, anyone can copy and paste your AdSense code on any site, even if you do not own or manage that site. And as we know, the AdSense TOS does not allow all sites to include AdSense and some publishers can be banned if they do not comply. This feature allows AdSense publishers to control that - by saying, I only approve specific sites.
Forum discussion at WebmasterWorld.