I really struggled getting 3.0 "as a network" running. The installation guides unfortunately aren't clear enough about a few minor things for a new wordpress user like me. This resulted in lots of white screens! But with some perseverance here is what I worked out to get it working. Hope it works for you too!
First you need to read this....
(PS. I added a tiny edit to it)
You will also get your wordpress installation telling you what to do when you start creating a network. Read this closely too.
So thats two sources of info. Factor that along with my workflow below and you should figure it all out after a couple of tries perhaps! ;)
1. Install 3.0 like normal wordpress
2. Log in to admin panel and stay logged in
3. Using CPanel's file manager in a different browser window, edit your wp-config file and add define('WP_ALLOW_MULTISITE', true); just above where it says /* That's all, stop editing! Happy blogging. */:
4. Click on any button, perhaps add a page or any other function to "refresh" your admin panel. Then cancel that selection.
5. What will happen is under Tools, a "network" option will appear under Export
6. Click on it and follow the instructions
7. Again, whilst still staying logged in to your admin panel, go to a different browser window and use CPanel's file manager to edit the .ht access and wp config file and also create the required directory you were instructed to do by the installer
8. What the earlier instructions failed to mention is that when we now place all that code into our wp-config file, we need to also REMOVE our define('WP_ALLOW_MULTISITE', true); line from wp-config which we had earlier inserted.
9. Save these two files. Check your directories permission is writeable
10. Log out of your admin panel
11. God bless, you can now log back in :)