Is it not the ui.draggable?
If you go here (in Firefox and assuming you have firebug) and look in the firebug console youll see I am doing a console.dir of the ui.draggable object which is the div being dragged
Therefore the code you need in the drop function is
drop: function(ev, ui) {
//to get the id
//ui.draggable.attr('id') or ui.draggable.get(0).id or ui.draggable[0].id
console.dir(ui.draggable)
}