Due to rise in hacking of websites/blogs, Google Webmaster Tools plan to run a test to alert webmasters if their content management system (CMS) or publishing platform have a security hole or be hackable. They’ll be alerting five to six thousand webmasters of potentially vulnerable sites by leaving messages in the Message Center. If you manage a website but haven't signed up for Webmaster Tools, the messages’ll be saved and if you sign up later on, you'll still be able to access any messages that Google has left for your site.
Google starts testing with WordPress (2.1.1) that is known to be vulnerable to exploits. If the test goes well, they may expand these messages to include other types of software on the web. The message that a webmaster will see in their Message Center if they run WordPress 2.1.1 will look like in the image: