function wpse32738_get_editable_roles( $editable_roles )
    {
        global $wp_roles;
        if ( current_user_can( 'manage_options' ) ) {
            unset($role_names['administrator']);
        }

        return $editable_roles;
    }
    add_filter( 'editable_roles', 'wpse32738_get_editable_roles' );