After upgrading from 2.9.2, to 3.0 beta 2, to 3.0 RC1, then enabling multisite, and changing a new subdomain blog's siteurl to http://www.mysite.com, then attempting to access new site's Dashboard via Super Admin, it brings you to wp-login.php for http://www.mysite.com, and even after logging in, returns to wp-login.php. In other words, new site's Dashboard is unreachable. Dashboard for root site is reachable, and upon changing the siteurl for http://www.mysite.com back to mysite.rootdomain.com, Dashboard is reachable for secondary site. Tried everything here, but this issue is certainly not resolved:
I tried creating new users as admins on the new domain, and regardless, when the secondary domain is set to http://www.mysite.com, the Dashboard is unreachable.