On older Wordpress installs (pre-3.0 I believe), you couldn't change the username of the first user from "admin" when setting up a blog, and you had to manually change it later. Yes, it was stupid.
I remember having to perform some magical incantation to actually pull that off around then. Set up WP, log in, create new user, set it as admin, log in as the new user, try to delete the admin account, log back in as admin because you forgot something, log in as the new user again, actually delete account.
No wonder everyone stuck with 'admin'.