Anonymous Coward

I’m not sure if it’s different with 1.5, but for 1.2, you edit the function comment_author(), which is in the file

wp-includes/template-functions-comment.php

around line 107, change
echo "Anonymous";
to
echo "Anonymous Coward";

edit – they’re actually single quotes around the word, not double ones like it’s posted here..

comment-functions.php
line 143
function get_comment_author() {
global $comment;
if ( empty($comment->comment_author) )
$author = 'Anonymous';