This plugin by Jason Grim will upgrade your WordPress 3.0 blog to Multi-Site with the click of a single button.
One of the biggest changes in WordPress 3.0 over previous versions is the addition of multisite capability, allowing users to deploy a single WordPress installation across multiple sites.
The multisite capability is not enabled by default and requires users to manually edit the “wp-config.php” file to start the feature. Multisite is also restricted to providing site addresses within the main domain of the site in which WordPress 3 is installed.
This plugin does away with the need to edit any files.
Plugin homepage: http://jgwebdevelopment.com/plugins/wordpress-multi-site-enabler-plugin