I personally do not see it, and when I go to the URL, it takes me to a template page with no features.
What does site authentication do in AdSense?
In short, it allows you to feed AdSense a username and password, so that they can access pages that are secured by a user/pass requirement.
Typically, when AdSense can't see a page, they will serve up an ad that is related to the overall site theme or just serve up a public service ad that doesn't pay well or at all.
ProBlogger reported they began testing this in November 2006. Here is the email some beta testers received:
You are one of a select group of publishers that has been chosen to try out our new Site Authentication feature. Site Authentication allows publishers to give our crawler access to login-protected content so that users can receive targeted ads. While there is no obligation for you to try out this new feature, you may find it helpful if you have pages of your site protected behind a login screen.
“As an example, an online newspaper site may protect content behind a login for premium readers. Without Site Authentication, the publisher can’t show readers targeted ads on those premium pages because our crawler has not been able to access the content. Using Site Authentication, the publisher can now specify details for our crawler to pass the login screen and reach the protected content, allowing AdSense to serve relevant ads to the readers.
So does this mean it will be launched publicly soon? I am not sure.
Forum discussion at DigitalPoint Forums.
Update: DaveN has a screen cap at his blog, so check it out.