The proximity sensor on the iPhone 4 is used to disable the screen from errant touches when you hold the phone to your ears.  Basically, it senses when your ears are close to the phone, and turns off the touch screen so your ears can’t hit the options on the screen.  This method has worked great for the iPhone 3GS.

But, with the iPhone 4, many have been complaining that the proximity sensor isn’t working properly – and the phone’s screen turns live when it’s next to your ear.  I can personally attest to having this problem a few times – I even made an appointment with the Genius Bar at an Apple Store to figure it out.  Their response was the new software update should fix the problem – but the folks over at the popular blog TUAW got a different response when they wrote: Proximity sensor woes caused by reflective ear canal.

They were told the following by one of the Apple Genius Bar employees:

Knowing others were having issues with their proximity sensor, I made an appointment at my local Apple Store with one of the esteemed “Geniuses.” His name will remain redacted but I swear he stifled a laugh when he told me the cause of the problem. Apparently, the re-location of the proximity sensor in iPhone 4 causes the sensor to be more likely to be triggered by light “bouncing around the ear canal.”

Light bouncing off the ear canal? I doubt it. Using an otoscope to look at the ear drums isn’t the easiest thing in the world, and requires a bit of adjustment at that. I doubt “reflective ear canals” are even possible, there isn’t a straight view from your ear drums to the external portion of your ear. Needless to say, a physical exam finding of “shiny pearly white eardrums without effusions” probably isn’t going to trip the proximity sensor anytime soon.