Ah, ok, I see what's happening. Here's what you want to do (working example, here: http://jsfiddle.net/mori57/L3CF8/1/)
$(".droppable").droppable({
tolerance: "pointer",
drop:function(){
// You need to tell your sortable to cancel
// the last drag/drop interaction
$("ul").sortable("cancel");
alert("Do something on drop ...");
}
});