I just came accros this:
http://wiki.geeklog.net/wiki/index.php/SoC_improving_comments
"Optionally send a notification email when a reply has been made to a user's comment
(for registered users only)"
Hm, the wordpress plugin called 'Subscribe to Comments'
http://txfx.net/code/wordpress/subscribe-to-comments/ also manages anonymous users and identifies them by email. This is one more step user friendly.
As for me, I`d like that too