Google has added a new tab, a tab they have been beta testing for a couple months, named Site Diagnostics. What this tool does is show you which pages the AdSense crawler is having problems getting to. Why would they crawler have a problem getting to those pages? The several possible reasons include a robots.txt file blocking then, password protected pages, server down or slow and other reasons explained in the AdSense help pages.
I have posted screen captures at the Search Engine Roundtable.