This security update is so easy to fix you need to just do it now. Head over to the Wordpress 2.6.3 update page, read the details on the vulnerability in the Snoopy library and the download 2 files
http://wordpress.org/development/2008/10/wordpress-263/ You can then drop class-snoopy.php and version.php directly into your wp-includes/ folder and be up to date.

When the security issue is small or large, the ability to do such quick and easy updates makes it a must do for every Wordpress administrator on their weekend to do list.