I installed WordPress 3.2.1 fresh on a new domain, and then enabled Multi-Site, which functioned as documented. I could reach /wp-admin/network without problems.
Comparing the database of this WP with the problem installation, in the wp_sitemeta table, admin_email was not set for the problem site.
Also, wp_sitemeta > site_admins was different. The problem website had it set to:
a:1:{i:0;N;i:1;s:3:"username";}
The correct value was:
a:1:{i:0;s:3:"username";}
Setting admin_email to my Admin account’s email and changing siteadmins to the correct value above added “Site Admin” to the top right drop down box under my username.
However, when trying to load the Site Admin section, my browser produced an error:
This webpage has a redirect loop. The webpage at
[baseurl]/wp-signup.php?new=[baseurl] has resulted in too many
redirects. Clearing your cookies for this site or allowing third-party
cookies may fix the problem. If not, it is possibly a server
configuration issue and not a problem with your computer.
After I added www. to:
define('DOMAIN_CURRENT_SITE', 'www.example.com' );
in wp-config.php, I was able to load /wp-admin/network without problem, and all network / multi-site options were visible.