A Google Webmaster Help thread has confirmed reports of an issue with authenticated your site via Google Webmaster Tools to allows AdSense's bot to crawl password protected sections of the web site.
One of the steps of authenticating AdSense to allow the AdSense crawler to access pages that are behind a login is to verify your site with Google Webmaster Tools. The data feed to pass over the authentication authorization between Webmaster Tools and AdSense was offline for a few days.
Dennis G. from the Google Webmaster team said:
Our data feed from Webmaster Tools to AdSense slowed down significantly for a few days. It's getting back to normal now; please try again, and post back here if it's not working by tomorrow. Sorry for the inconvenience.
We have not had any real confirmations from publishers that this has been fixed, but hopefully it has.
Forum discussion at Google Webmaster Help.