It dawned on me that some of the proximity sensor parts are not installed. Was the glass broken before the conversion took place? If you look REALLY closely at the proximity sensor, you should see a VERY small black square around it. It pops off and you'll see it is actually attached to a very small piece of colored glass. If this is missing, the proximity sensor will not function correctly. Most likely because it will pickup reflect light from the LCD. Hope this helps.
iPhoneAid removes the mylar(silvery)piece that covers the holes in front of the sensor on the front white glass replacement. On a black to white conversion, it appears to work very well in most cases. I should mention that we are using 4.0.2 OS with the conversion.