<?php if ('open' == $post->ping_status) : ?>
   <h3 id="trackback-url"><?php _e('Trackback URL', 'hoge') ?></h3>
   <input onfocus="this.select()" readonly="readonly" value="<?php trackback_url(); ?>"
       class="tb_url" id="tb_url" name="tb_url" accesskey="x"/>
<?php endif; ?>

input#tb_url {
  〜
}