Today we updated the admin module in Drupal 6 to 6.x-2.0-beta5, the latest version. This module is responsible for the majority of administration interfaces on the sites we build and run, so it's likely your site was updated as a result of this change.
You don't need to do anything, and you probably won't notice any difference in the way your site or its administration interface works; the changes are behind-the-scenes and address security issues in Internet Explorer that could have been used maliciously.
If you would like to know more about how your site has been affected, or security on your site in general, do get in touch using the contact details at the bottom of this site or use the contact page.