very nice! somehow I've missed this mod...
Just yesterday I learned about gravatar site, and thought it was a good and innovative idea/concept.
In addition, if you don't want to see gravatar icon if no avatar available, you can add:
&default=path to default avatar image
at the end of image address.
i.e.: $comment_user_gravatar = "<img src='http://www.gravatar.com/avatar.php?gravatar_id=$md5&size=$default_size&rating=$default_rating&default=".$script_url."/".TEMPLATE_PATH."/images/user_online.gif' width='$default_size' height='$default_size' alt='' />";