All self-hosting wordpress bloggers have to upgrade whenever a new version of WordPress is released, not only to get new features and bug fixes, but also eliminate security loophoes easily exploited by hackers to hack your blog. Now Google webmaster tools will warn blog owners when their old wordpress versions are prone to hacking.
Google is presently running a limited test and warning webmasters who are hosting WordPress 2.1.1, which was known to be vulnerable to exploits. Remember how a cracker gained user-level access to one of the servers that powers wordpress.org, and modifed the wordpress 2.1.1 download files. They say if the test goes well, they will expand these messages to include other types of software on the web.
Here is a sample message which affected users will get.
I think this is a good way to warn wordpress bloggers about old versions at high security risk. But when someone has not upgraded wordpress for over a year, there is a high chance they have become inactive and stopped blogging. Upgrading wordpress is an essential task for wordpress bloggers (one of the benefits of hosting on WordPress.com). Anyone can find out your wordpress version easily, so it is recommeded you drop version string in header.php. Stay safe with the latest version of WordPress.