actually, they are most likely using part of the new drag and drop API in HTML5. firefox and chrome both provide a `ev.originalEvent.dataTransfer` object as part of the DragEvent interface. additionally (and importantly), they allow the drag/drop to originate from outside of the browser.
see: http://decafbad.com/blog/2009/07/15/html5-drag-and-drop
and: http://hacks.mozilla.org/2009/07/html5-drag-and-drop/