[WPMU] is most famously used for WordPress.com where it serves tens of millions of hits on millions of blogs each day. Many bloggers interested in using WordPress MU already have a WordPress blog. Unfortunately, I think it is not possible to turn WordPress installs into WordPress MU installs directly uploading new files and upgrading.

The post includes a 17-step process you'll need to follow to get your current WordPress installation migrated over intact (with a few sub-steps along the way). At the end, there's a brief tour of the new panels you'll have for things like the blogs, users, themes and administration.