WordPress 3.0 is when the big merge happens. The core of WPMU is incorporated in WordPress which means that you can now run multiple blogs from one installation of WordPress. This new feature is being called Network or Multi-site option. By default this feature is disabled. If you want to use this option in your WordPress, then you would have to enable it.
To enable this feature, simply open your wp-config.php file located in your WordPress root folder. Then add the following line anywhere in the file:
Once you have done that, you should be able to access the network page from Tools » Network
Now, you no longer need to install multiple copies of WordPress because you can run multiple blogs from one installation. By default, you would only be allowed to run other blogs as a subdomain. To use other domains for blogs, you can use a Domain Mapping plugin.