Clint, the os.system call will set the environment variable but env variable settings only affect new Desktop apps when they start up. It will have no effect on the current environment within your script, or any standalone Python scripts.
Note, arcpy.SetProduct() has no effect - it's been left there so old scripts won't be broken but it doesn't do anything.
Luke is right - the way to get this to work correctly is to import arcview ("ArcGIS Basic") before arcpy.
import arcview
import arcpy
sa_status = arcpy.CheckOutExtension("spatial")
arcpy.AddMessage("ArcGIS product: " + arcpy.ProductInfo())
arcpy.AddMessage("Spatial Analyst: " + sa_status) # "CheckedOut"
Help 10.2: Accessing license and extensions in Python