config.xml:

<adminhtml>
  <events>
    <sales_order_status_history_save_before>
      <observers>
        <module_status_history_save_before>
          <class>module/observer</class>
          <method>addUserNameBeforeComment</method>
        </module_status_history_save_before>
      </observers>
    </sales_order_status_history_save_before>
  </events>
</adminhtml>

Observer.php:

public function addUserNameBeforeComment($observer)
{
        $data = '<strong>' . Mage::getSingleton('admin/session')->getUser()->getFirstname() . ' ' . Mage::getSingleton('admin/session')->getUser()->getLastname() . ': </strong>';
        $history = Mage::app()->getRequest()->getPost('history');
        if ($history && isset($history['comment']) && $history['comment'] != '') {
            $history['comment'] = $data . $history['comment'];
            Mage::app()->getRequest()->setPost('history', $history);
        }
}

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