If you have been running WorPress MU for a while, you may not have seen some of the recent additions to values defined in wp-config.php. For single site WPMU installs, as of 2.6.1, you can save a couple of database and/or internal cache lookups by adding 3 defines to your wp-config.php:
define('DOMAIN_CURRENT_SITE', 'domain.tld' );
define('PATH_CURRENT_SITE', '/' );
define('BLOGID_CURRENT_SITE', '1' );
Substitute your WPMU install’s domain and path for domain.tld and /. It is a small optimization, but every little bit helps.