You can use dojo.subscribe
to do something when a drop is finished like so:
dojo.subscribe("/dnd/drop", function(source, nodes, copy, target) {
// do your magic here
});
There's examples of using subscribe on the dojotoolkit tests site. More info about dojo publish and subscribe too.
Alternately, you could connect to the onDndDrop
method.
var source = new dojo.dnd.Source( ... );
dojo.connect( source, "onDndDrop", function( source, nodes, copy, target ) {
// make magic happen here
});
connect methods are called at the end so the items will be there at that point.