WP 3.0 new site not working

Hi,
I installed WP 3.0 and I follow all the instructions (modifying wp-config.php and .htaccess), to activate the network functionality.
Now when I try to create a new site (a blog) in the superadmin panel it says that it’s OK, but I don’t find anything.
In the public part (the site) I have “Index of /” and in the private part (the wp-admin) I have 404 “Not Found”.
What should I check?
Thanks!
Paolo