I’m pretty sure you need to edit the wordpress/includes/pluggable.php file. Here’s the chunk of code you’re looking for:
@wp_mail(get_option('admin_email'), sprintf(__('[%s] New User Registration'), get_option('blogname')), $message);
if ( empty($plaintext_pass) )
return;
$message = sprintf(__('Username: %s'), $user_login) . "\r\n";
$message .= sprintf(__('Password: %s'), $plaintext_pass) . "\r\n";
$message .= site_url("wp-login.php", 'login') . "\r\n";
wp_mail($user_email, sprintf(__('[%s] Your username and password'), get_option('blogname')), $message);
Just remember to have a backup of the original file! I modified it once and it screwed my website! It didn’t show up. It looks like the php version of your server checks for errors in this file, so be careful with it ??