theme_lt_username_title can not be overwritten in template.php

Just wanted to drop a quick note that this theme function can not be overwritten in template.php...

I am writing like so:

function garland_lt_username_title($form_id) {
  switch ($form_id) {
    case 'user_login':
      // Label text for the username field on the /user/login page.
      return t('Email');
      break;

    case 'user_login_block':
      // Label text for the username field when shown in a block.
      return t('Email');
      break;
  }
}

No errors come about and no changes take place.