I didn't see any mention in the iPhone 4S teardown about the proximity sensor.
I've had my iPhone 4 replaced 3 times, first for the proximity sensor problem, then for the yellow screen, finally because it wouldn't install IOS 5.
The second replacement phone also had intermittent problems where my ear would put the phone on mute, or start 'dialing' while I was on a call.
I'm really interested to find out if they did anything about this with the new phone.
Hmm. I came here to try to figure out what the mysterious extra window is which appears in photos of the white iPhone 4, above the "receiver" ear slot. I think it's for a light sensor, and I assumed that it was also there on the black model, but just not visible because of the black surround.
I think I can see it on my phone by using an LED flashlight to provide lots of illumination, but I'm not sure, and it doesn't seem to be there from the back in these photos.