The Google Webmaster Central Blog announced they are stepping up their notifications within Webmaster Tools. They will now send “SiteNotices” for a “significant increase” in unreachable content, that is when GoogleBot cannot reach a number of pages on your web site.
Here is a picture of what a SiteNotice may look like:
Google will only send these out if you have a verified site in Google Webmaster Tools and they recommend setting up the email forwarding option to make sure you don’t miss these important alerts.
As you know, if GoogleBot cannot access your content for some reason, that can be a big issue for your visibility in Google’s search results.