arcgis 10.0 - Adding Layer in current session using ARCPY

I wanted to add a layer through the toolbox using arcpy. The below script works fine through the python console in ArcMap, but does not when executed through the toolbox. The layer flashes for a second in the layer management window and disappears. This looks very unlikely and I am not able to figure out how it could not work in the toolbox. Let me know if I had missed something.

import arcpy

mxd = arcpy.mapping.MapDocument("CURRENT")
df = arcpy.mapping.ListDataFrames(mxd, "*")[0]
sourceLayer = arcpy.mapping.Layer('C:/WIP/Shape files/KARTHIK.TEST_TABLE.lyr')
arcpy.mapping.AddLayer(df,sourceLayer,"TOP")
arcpy.RefreshActiveView()

Answer

What version / service pack do you have? It seems this problem was fixed recently in v10 SP1.

http://forums.arcgis.com/threads/8331-AddLayer-only-temporarily-adds-the-layer-to-map

You can try downloading SP1 here. http://resources.arcgis.com/content/patches-and-service-packs?fa=viewPatch&PID=66&MetaID=1685