"Recently we've seen more websites get hacked because of various security holes. In order to help webmasters with this issue, we plan to run a test that will alert some webmasters if their content management system (CMS) or publishing platform looks like it might have a security hole or be hackable. This is a test, so we're starting out by alerting five to six thousand webmasters. We will be leaving messages for owners of potentially vulnerable sites in the Google Message Center that we provide as a free service as part of Webmaster Tools. If you manage a website but haven't signed up for Webmaster Tools, don't worry. The messages will 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."

http://googlewebmastercentral.blogspot.com/

[moved from "Google Message Center warnings for hackable sites introduced". Thanks Hebbet!]

From Google Webmaster Central Blog:
http://googlewebmastercentral.blogspot.com/2008/10/message-center-warnings-for-hackable.html