This might work, although I don't know if there is a better or more appropriate way to do it. It looks like the key is to make a layer file first and then add it with arcpy.mapping. It looks like you could make a layer file using the Make Feature Layer tool and the Save Layer to File tool. Then, you can probably use the AddLayer function like in the 10.0 sample code I found below:
import arcpy
mxd = arcpy.mapping.MapDocument(r"C:\Project\Project.mxd")
df = arcpy.mapping.ListDataFrames(mxd, "New Data Frame")[0]
addLayer = arcpy.mapping.Layer(r"C:\Project\Data\Orthophoto.lyr")
arcpy.mapping.AddLayer(df, addLayer, "BOTTOM")
mxd.saveACopy(r"C:\Project\Project2.mxd")
del mxd, addLayer