My previous iphone 4 (which was a replacement of the original phone due to a data issue which was the carrier's fault in my area, but which they took over 6 weeks to fix), suffered from proximity sensor issues when updated from 4.1 to 4.2.1. Unfortunately I could not revert to 4.1. I struggled with the issue for a month, posting several times in the megathread on the apple discussion boards.
I finally gave up and went to the Apple store and they checked the serial number and advised that it was within range. However, my phone I am probably certain was a refurbished one (nothing wrong with that) and its refurbishment date was late December. With refurbishing, I know the back and front are replaced but I do not know if the sensor itself is replaced. With this phone I had the problem- putting person on speakerphone - about every 4th call.
My replacement phone - I am pretty sure as well a refurbished one - was manufactured in the 8th week of 2011, late enough one would think to have had the manufacturing/component issues of the proximity sensor sorted out.
Not the case and I still have the issue but less frequently. I can go 10 calls in a row and not have the problem and then it will bite me. It is always the speakerphone being activated.
It is such a crapshoot with these phones that I doubt I am going to go to the Apple store again. My screen is good, no overly yellow tinge, no flash issues etc. I am concerned that exchanging this phone may result in my getting a phone with either the same issue or different issues.