While the iPhone has generally shown pretty robust in extreme weather condition, it can sometimes still get hurt, on the inside.
Indeed, to check whether your iPhone has stopped working because you dropped it into some liquid, Apple planted three white sensors in your iPhone; one in the headphone jack, one in the dock connector and one inside, on the motherboard.
The first thing App Store Geniuses or any iPhone repair service normally does is to check them out, if your iPhone was in contact with some liquid, the sensors turn from white to red and this mean your warranty is gone.
This is what happened this winter to a couple Polish iPhone owners. They received their iPhone back from repair with a notice saying their iPhone had been in contact with liquid and won't be repaired. Since most of them had taken good care of their device and were convinced their phone hadn't been in contact with any liquid or snow, they tried to figure out what had happened.
Well, what they discovered is particularly interesting. The iPhone does indeed work just fine in the atmospheric condition prescribed by Apple, but the sensors might tell another story. If held in the cold too often and exposed to temperature changes, your liquid contact sensors can get affected and make you lose your warranty. It won't prevent your iPhone from functioning, but next time you need servicing, you'll be out of luck.
Alright, Polish winter happens to be particularly tough, but still, take care of your iPhone.