There are so many great things about WordPress 3.0 (like the awesome Menus feature for example), and one of the best is that they’ve combined WP and WPMU at long last. You have to turn on the multi-site option by adding a bit of code to the wp-config.php file to enable it:
define('WP_ALLOW_MULTISITE', true);
After adding this, you should see Network under the Tools options in the Admin panel and you can take it from there!