(It's been a long time since I've done this, and I'm doing it from memory and a glance at the docs. If this doesn't work, let me know and I'll double check w/ code.)
In draggingSession:sourceOperationMaskForDraggingContext:
you should include NSDragOperationDelete
as one of the legal operations. You will then receive NSDragOperationDelete
back in your draggingSession:endedAtPoint:operation:
to indicate that the item was dropped on the trash.