If you were anywhere near me, I would gladly show you not one, but two live demos of it. I bought iPhone 4's for my wife and I last week, and both of them have the proximity sensor issue - "cheek dialing" keypad numbers, attempting to initiate Face Time calls, muting calls, hanging up, etc. It's not a "user error" issue....I've owned a 3G for well over two years and never once had the issue with it...I know how to hold and use an iPhone when making phone calls. It's not an accessory issue - it happens with both phones, both with and without cases (and neither of us have screen protectors). It's a hardware and/or firmware issue, plain and simple. Steve Jobs has publicly acknowledged it and said they're working on a fix for it, so it's obviously "real pervasive" enough to warrant Apple's attention.Or me...and the team I support...still looking for my first live demo of prox sensor issue...not saying it isn't real, just not real pervasive in my experience.
I'm not saying every iPhone 4 has the issue, but it certainly seems prevalent enough to assume that it's a significant issue. I'm not one to get all hysterical about a phone - it's just a phone - but it definitely is an irritating issue, and one I'd be very glad to see fixed.