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!
This is really cool – I have upgraded to WP 3.0 but didn’t know about this feature. Why do you think they made it so hidden, though?
I don’t know that it’s “hidden” per se. It is a bit more complicated than I would have liked, and I imagine it will be easier with future versions, however using multi-site is not just a simple option. It affects the entire structure of the set up, so it’s not surprising you have to make a change in the code.