On the DataGrid that's the recipient of the drop, you can
listen for the dragDrop event:
<mx:DataGrid dragDrop="handleDrop(event)" ... >
The handleDrop function has a event of type DragEvent. When
the handleDrop function gets called, the event contains a
DragSource property. Look up the DragSource class in the Flex docs.
You'll find that the drag initiator has stored its data under a
named format, typically "items" for DataGrid and lists. All you do
is ask the DragSource for the data associated with the format.