Customize the text before the comment form in WordPress. Note: Logged in users will not see this text.

comment-notes-before.php

<?php //remove this line

add_filter( 'comment_form_defaults', 'cd_pre_comment_text' );
/**
 * Change the text output that appears before the comment form
 * Note: Logged in user will not see this text.
 * 
 * @author Carrie Dils <http://www.carriedils.com>
 * @uses comment_notes_before <http://codex.wordpress.org/Function_Reference/comment_form>
 * 
 */
function cd_pre_comment_text( $arg ) {
  $arg['comment_notes_before'] = "Want to see your ugly mug by your comment? Get a free custom avatar at <a href='http://www.gravatar.com' target='_blank' >Gravatar</a>.";
  return $arg;
}