Proximity sensor issues are probably a mix of user error and maybe a limited number of manufacturing defects. Never had a problem with either of our iPhone 4s from launch, nor the ones I've tried at the local Apple stores or friends iPhone 4s.
I don't buy the "user error" thing at all in regards to the proximity sensor issues. I've personally seen three iPhone 4's that exhibit the issue - two of them (a friend and I) both owned 3G's before and never had any PS problems with them; the other one (my wife) has never owned an iPhone, so didn't have the opportunity to pick up any "bad iPhone technique" from previous experience.
FWIW, all three of us have found that the 4.1 upgrade helps somewhat, but doesn't entirely eliminate the issue. I think it's ridiculous, condescending and insulting to infer that iPhone users with PS issues are too stupid to know how to hold/use an iPhone. Making a phone call isn't rocket science.
For the record, I'm not one of the OCD whiners who inspects my iPhone with a magnifying glass looking for dust. I couldn't care less that the Calendar app now uses pastel colors, don't pull my case off twice a day and wipe everything down to make sure I don't get scratches on "the precious"; I don't shoot photos in near-darkness looking for green blobs, nor do I scrutinize every aspect of every app looking for obscure, difficult-to-reproduce bugs. I don't have dreams about scratches on my phone, and I don't lie awake at night obsessing over whether or not my iPhone suffers from every issue posted in MacRumors threads or internet blogs. Overall, I think the iPhone 4 is a great device and I'm very happy with it - but I do strongly believe that the proximity sensor issue is a hardware/manufacturing defect, and is undoubtedly the iPhone 4's one "Achilles' Heel". It's the one thing about the phone that irritates me, even though I rarely have to deal with it since I use a bluetooth earpiece most of the time.
The only thing more irritating is people who insist that there is no issue with it, and that a large number of people who have owned and used previous versions of the iPhone without issue are suddenly too stupid to know how to properly hold a phone.