Primary administrator pre-1.7
From MoodleDocs
The Primary Administrator is the administrator who has the lowest user id (typically the earliest created admin user).
The 'primary administrator' is the only one that can assign (or remove) other users as administrators. The primary administrators email address will receive all returned email ('bounces') for undeliverable addresses and also, other admins are unable to edit the primary admin's profile.
It appears that the only way currently to change the primary administrator is by changing the database directly (see the forum discussions linked below for more info).
