function comment_author_link_new_window() {
global $comment;
$url = get_comment_author_url();
$author = get_comment_author();
if (empty( $url ) || 'http://' == $url)
$authorlink = $author;
else
$authorlink = "<a href='$url' target='_blank' rel='external nofollow' class='url'>$author</a>";
return $authorlink;
}
add_filter('get_comment_author_link', 'comment_author_link_new_window');