When I had to take in my phone for repair due to a faulty power button I ran this program: RUU_SuperSonic_S_Sprint_WWE_3.70.651.1_Radio_2...e xe
from another thread. I didn't check extensively as to whether it truly unrooted the phone but at least on the surface it seemed to. The Sprint techs said nothing when I brought it in. You just download the program and plug in the phone when it tells you and it takes about 10 minutes.
Then they gave me a new phone because they couldnt fix the button
Great thread guys!