This is by design, the temporary graphic used in editing is only accessible at the API level. What you have access over is the original graphic. If the reason you want access to the edit graphic is to customize the symbols used in editing, you can use EditGeometry instead as shown in this sample: http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#EditToolsGeometry. You can download WPF SDK to see the WPF version.