We reported in early May that Google was beta testing an AdSense feature named site diagnostics tool. Now, it appears that Google has released it to all AdSense publishers. If you login to your account, you should see a new tab on the first tab named Site Diagnostics. You can see a full size version by clicking here.
What does it mean? Well if you have URLs that are blocked it will show up in that report. The AdSense help says, "Your URL might show up as blocked in the site diagnostics tool for a number of reasons." There may be a robots.txt blocking the crawler, it may be password protected, your site may be down, etc. Any time you see something reported there, there has been a crawl issue to the page. I would guess this also can be used to determine your site crawl health for normal web search spiders.