bernardpeh has posted some useful info and a video “How to Host Multiple Sites in WordPress 3.0” on Sitepoint.
“This is great news for anyone who has multiple blogs hosted on different domains. Combining all the blogs into one install not only saves disk space, it also makes maintenance and upgrades easy. If you’re using WordPress version 3.0 and above, this feature is already built in for you. Unfortunately, it’s not turned on by default. ”
One line needs to be added to your wp-config.php file.
Read the full article at: http://blogs.sitepoint.com/2011/02/08/video-how-to-host-multiple-sites-in-wordpress-3-0/
Install instructions on WordPress Codex:http://codex.wordpress.org/Create_A_Network