<script type="text/javascript">
$(function(){
    var itemSel=$('#item_select');
    itemSel
    .attr("prevValue",itemSel.val())
    .change(function(){
        var o=$(this);
        if(o.val()!='未选择'){
            o.attr("prevValue",o.val());
        }else{
            o.val(o.attr("prevValue"));
        }
    })
})

</script>
<select name="select" id="item_select">
  <option value="未选择">未选择</option>
<option value="a" >a</option>
<option value="b" selected="selected" >b</option>
</select>