<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.2.6/jquery.js"></script>      
      <script type="text/javascript">      
      $(document).ready(function(){      
      $("#timeg").find("a:contains('$USERNAME$')").html("$USERNAME$ (это вы)");      
      });      
      </script>

Заменить $ONLINE_USERS_LIST$ на <span id="timeg">$ONLINE_USERS_LIST$</span>