Everyone's mileage may vary, but here is what my latest genius bar appointment gleaned.
The tech I got didn't know anything about it and had me recreate the issue which I did quite quickly. After she initially told me that I was 'holding it wrong'..I seriously cracked up...and that the sensor means I have to be more careful with it, I barely moved it away from my ear and it happened..she then agreed it was a problem. She asked me to try it on her phone and I did..recreated the problem.
She went to the 'back' and said that when she told others, they all told her that 'oh yeah, the proximity sensor problem'. She came out and reported that the collective brain trust in the 'back' said that it is a known issue at Apple (duh..there's been some press and these forums about it)...and that it is a confirmed HARDWARE problem. She has a new 'finished good' unit ordered for me. Of course they didn't have any in stock. So, I am a litle leary because none of the other odd things are happening with my phone that are happening with others..no yellow screen, no bad bluetooth, no hot phone, no crashing ipod, no other 'one off' things. So, all I have is the signal issue and proximity sensor...so I was hoping for a non hardware fix because I hope I don't get one and all the problems now creep up. So, that's the current story from St. Louis...they say hardware and a replacement is the fix. Good luck to all..it is a super annoying problem.