All DragVisualProviders for the RadListBox, as described in http://www.telerik.com/help/silverlight/radlistbox-features-dragdrop.html, show as a bare minimum a horizontal line that clearly shows where the drop within the list will occur.
Unfortunately the RadGridView does not even have that out of the box for its DragVisualProviders, so dragging elements within a RadGridView provides no indication at all to where the drop will happen! This seems like a very basic feature the RadGridView should have to support dragging/dropping within it (which also seems like a very common functionality).