<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>