How can I find the new div id of an element moved with Jquery sortable?

I want to know the new position (div id) of an element I just moved

How can I do this ?

http://jqueryui.com/demos/sortable/#empty-lists

<script>
    $(function() {

        $("#sortable").sortable({
    update: function(event, ui) {
        alert("New position: " + ui.item.index());
    }
});

        $( "ul.droptrue" ).sortable({
            connectWith: "ul"
        });

        $( "ul.dropfalse" ).sortable({
            connectWith: "ul",
            dropOnEmpty: false
        });

        $( "#sortable1, #sortable2, #sortable3" ).disableSelection();
    });
    </script>


<ul id="sortable1" class='droptrue'>
    <li class="ui-state-default">Can be dropped..</li>
    <li class="ui-state-default">..on an empty list</li>
    <li class="ui-state-default">Item 3</li>
    <li class="ui-state-default">Item 4</li>
    <li class="ui-state-default">Item 5</li>
</ul>

<ul id="sortable2" class='dropfalse'>
    <li class="ui-state-highlight">Cannot be dropped..</li>
    <li class="ui-state-highlight">..on an empty list</li>
    <li class="ui-state-highlight">Item 3</li>
    <li class="ui-state-highlight">Item 4</li>
    <li class="ui-state-highlight">Item 5</li>
</ul>

<ul id="sortable3" class='droptrue'>
</ul>

In you callback, you can use event.pageX and event.pageY to determine the offset with respect to the page.