Display commenter’s registration date on comments?

In your wp_list_comments callback function, you can call get_userdata to get any additional comment author data:

$userdata = get_userdata( $comment->user_id );
echo 'Registered: ' . $userdata->user_registered;

// format the date
// Sunday January 13th 2013
echo 'Registered: ' . date( 'l F jS Y', strtotime( $userdata->user_registered ) );