so you're trying to figure out which draggable hit your droppable, right?
onDrop will automatically get the element that was dropped on it... then it's just a matter of getting the id. see below. identify() is part of prototype.
Code:
onDrop:function(element){
Effect.Puff($(element).identify());
}
that is untested, but based off my own working code. if it doesn't get you closer try dropping an alert in there to see what is being passed.