Might want to take a look at this thread.
http://forum.xda-developers.com/showthread.php?t=560865
The thing with DUN is that if your phone uses the DUN NAI to connect then Sprint will be able to see that you are tethering.
I would modify the DUN NAI to the regular NAI in EPST to be sure, however there are some risks to messing with EPST.