I've done afresh install of WP 3.0 several times, sometimes from trac and sometimes from the nightly build. Right now I am stuck.
After installing and logging in I goto Tools> Network and set it for subdirectories and then I get the following warning:
Theme Twenty Ten with 6 Widgets
Fatal error: Call to undefined function get_dirsize() in /home/xxxxx/xxxxx/www/wp-admin/includes/ms.php on line 445
The page quits loading there.