Google has released a new feature that allows the AdSense Crawler bot to index pages in password protected pages. This will allow you to have content focused advertisements on the password protected sections of your site. This will most likely mean more clicks, and thus more revenue from that advertising position.
Here are some details from Google’s AdSense blog:
We’ve recently launched a new feature called Site Authentication to take care of this problem. Using Site Authentication, you can give our crawler access to your login-protected pages by passing it information to log into your site. For example, let’s say your news site has a premium content area, with articles that only paying members can access. To get ads on those pages, you can use Site Authentication to provide our crawler with a test username and password. It’s an easy process that starts just by logging into your AdSense account and finding the ‘Site Authentication’ link under the ‘AdSense Setup’ tab. Once you’ve supplied us with a username, password, and a few other details, all you have to do is verify that you own the site through Google Sitemaps.
A very smart idea by Google, and something I think all sites that have areas that require user authentication to access will be using.