[melky]

$("#c").index()

пример :

<div id="container">
      <div id="a"> </div>
      <div id="b"> </div>
      <div id="c"> </div>
      <div id="d"> </div>
</div>
<script src="http://yandex.st/jquery/1.6.3/jquery.min.js"></script>
<script>
alert($("#c").index())
</script>