WordPress 3.0 includes new multisite features, meaning that it can run many blogs, even with their own separate domains, on one WordPress installation. This tutorial explains how to migrate multiple WordPress installations into one new WordPress 3.0 multisite. You can migrate sites that are using their own domain, as well as sites using a subdomain on your main domain.
In the video tutorial, Devin Price from WPTheming.com details the procedure necessary to switch a standard WordPress installation over to the multisite version so you can use multiple blogs from one installation.
The video is not in good quality, but you can read more onMigrating Multiple Blogs into WordPress 3.0 Multisite