I had an existing site that I tried to turn into a multisite WordPress by following the instructions at http://codex.wordpress.org/Create_A_Network. However, after going through the network set up business and logging in again, I get this message:
Please complete the configuration steps. To create a new network, you will need to empty or remove the network database tables.
I then deleted the tables associated with the set up (wp_site, wp_sitemeta, wp_registration_log, wp_blogs, wp_blog_versions) and tried again, which produces the same message.