DISCLAIMER:
I am not advocating the use of certain products. I am simply stating what worked in my specific situation.
Hi, everyone---
A few months ago I posted I experienced this problem with my iPhone 4 and removing an inexpensive third-party screen protector seemed to address the problem.
Unfortunately, the thing that got me functional again appears to be a little more complicated.
After removing the screen protector, I changed phone cases. Specifically, I put my OtterBox Defender case back on, because it has a built-in screen protector and I'm paranoid about scratching my screen.
After a week or two without any issues, I decided to remove my Defender case and go back to the case I was using when my woes began (a glow-in-the-dark LEGO-compatible case from SmallWorks). Much to my surprise and sadness, the problem returned. : (
I also have an OtterBox Commuter case, but I haven't bothered testing with that case yet.
All I can say for sure is that my phone doesn't face dial when I use my Defender case. My thought is that the case covers enough of the proximity sensor so it can function correctly.
Again, I'd like to reiterate that this is what worked for me. It might not (ok, let's be honest---it PROBABLY won't) work for you. Finally, this is simply a work-around. It still doesn't address the root cause that my prox sensor got flaky after an OS upgrade.
Thanks, and best of luck to everyone struggling with this issue.