AdSense crawling further

AdSense is testing selected group of password protected websites in a program called "Site Authentication" where they allow publishers to run Google AdWords at the sites where user authentication is required. Below is part of the email explaining the new AdSense program. ”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."