Just created a network from an existing installation of WordPress 3.0. Followed instructions *exactly* as written here. The main site URL is now redirecting to the structure http://www.mydomain.com/wp-signup.php?new=mydomain.com.
My goal is to have a main portal page (i.e., the main admin blog) show up at the base URL, and have sub-sites appear as subdirectories. I'm seeing older posts about this problem here and here, but I'm still stuck. Anybody got any ideas? Eventually, I want to be able to use the domain mapping plugin to map a domain to a subdirectory.