Seems like that isn't possible when working in a stand alone script. The following worked for me.import arcpy, os
arcpy.env.worksace = workspace = r"C:\GIS\scratch"
arcpy.env.overwriteOutput = True
mxd_string = r"C:\GIS\MXDs\empty_test.mxd"
fc = r"C:\GIS\someshapefile.shp"
mxd = arcpy.mapping.MapDocument(mxd_string)
df = arcpy.mapping.ListDataFrames(mxd)[0]
outlayer = "TEST_LYR"
layerfile = os.path.join(workspace,"ADDED_LAYER.lyr")
arcpy.MakeFeatureLayer_management(fc, outlayer)
arcpy.SaveToLayerFile_management(outlayer, layerfile, "ABSOLUTE")
addlayer = arcpy.mapping.Layer(layerfile)
arcpy.mapping.AddLayer(df, addlayer)
mxd.save()
del mxd
print "done"