Trying out the latest dev build on a test install running 3.0.1, and when I try to visit the supercache admin page, wp-admin/ms-admin.php?page=wpsupercache I get an error “Sorry, the WP Super Cache admin page only works on the main site of this network.” where main site is a link, which is trying to point to wpmu-admin…

And this is definitely the main site of the install.