Having your iPhone 4 replaced due to the Proximity Sensor issue is a gigantic waste of time. EVERY iPhone 4 has the potential to show the problem. It's just that not everyone has experienced it yet due to other variables, such as how they hold the phone, the size and shape of their ear, etc..etc.
I have two friends that were confident their iPhone 4s didn't have a problem. That is, until I made a call with their iPhone 4 and held it up to my face, carefully moved it around to find just the "wrong" spot, and BINGO... their iPhone 4 showed the same behavior. They could see the display coming back on and/or flicker on/off.
Ya'll are wasting your time getting your iPhone 4 replaced. The hardware is all the same. Apple will have to address this with a software/firmware fix.
Mark