You didn't even wait an hour before bumping. These kinds of feature requests are not life threatening, and as such we'll get to them when it's convenient for us. Be patient. Do not bump.
You could create a new plugin, using portions of subscribe2, and registered against a comment action hook (comment_post might be a good one).
Try this completely untested plugin. No support will be provided. You're on your own if you use it.
<?php
/*
Plugin Name: s2_auto_sub
Plugin URI: http://www.skippy.net/blog/
Description: Automatically adds commenters to the subscribe2 list of recipients
Version: 1.0
Author: Scott Merrill
Author URI: http://www.skippy.net/blog/
*/
do_action('comment_post', 's2_auto_sub');
function s2_auto_sub($comment_id = 0) {
if (0 == $comment_id) return;
$email = $wpdb->get_var("SELECT comment_author_email FROM $wpdb->comments WHERE comment_id = $comment_id");
if ('' == $email) { return $comment_id; }
if (is_email($email)) {
if (! $wpdb->get_var("SELECT id FROM $s2_table WHERE email='$email'")) {
$wpdb->query("INSERT INTO $s2_table (email, active) VALUES ('$email', '1')");
} // if !
} // if is_email
} // end s2_auto_sub
}