Must-use plugin to disable new user and password reset email notifications to administrators.

disable_admin_user_notifications.php

<?php
/**
  * Plugin Name: WP Disable Admin User Notifications
  * Plugin URI:  http://bitpiston.com/
  * Description: Disables new user and password reset notification emails sent to administrators.
  * Author:      BitPiston Studios
  * Author URI:  http://bitpiston.com/
  * Version:     1.0
  * Licence:     BSD
  */

// Disable password reset emails
function wp_password_change_notification() {}

// Disable new user emails
/**
 * Email login credentials to a newly-registered user.
 *
 * @since 2.0.0
 *
 * @param int    $user_id        User ID.
 * @param string $plaintext_pass Optional. The user's plaintext password. Default empty.
 */
function wp_new_user_notification($user_id, $plaintext_pass = '') {
    $user = get_userdata( $user_id );

    // The blogname option is escaped with esc_html on the way into the database in sanitize_option
    // we want to reverse this for the plain text arena of emails.
    $blogname = wp_specialchars_decode(get_option('blogname'), ENT_QUOTES);

    if ( empty($plaintext_pass) )
        return;

    $message  = sprintf(__('Username: %s'), $user->user_login) . "\r\n";
    $message .= sprintf(__('Password: %s'), $plaintext_pass) . "\r\n";
    $message .= wp_login_url() . "\r\n";

    wp_mail($user->user_email, sprintf(__('[%s] Your username and password'), $blogname), $message);

}