I have a iphone 4S, and I was wrong, the actual operating temperature is 0 - 35 C - that translates to 32F - 95F.
Here is what apple has to say:
Operate iOS devices where the temperature is between 0º and 35º C (32º to 95º F). Low- or high-temperature conditions might temporarily shorten battery life or cause the device to alter its behavior to regulate its temperature.
Store the device where the temperature is between -20º and 45º C (-4º to 113º F). Don’t leave the device in your car, because temperatures in parked cars can exceed this range.
iPhone, iPad, and iPod touch (4th generation): Keeping device within acceptable operating temperatures
Living in colorado, I am often outside in colder temperatures. I always charge my iphone before going out, and often when its cold (but not seriously cold) when I go to check my iphone its off, and won't turn on until I tuck it inside my jacket next to my skin and warm it up. Then it comes back on.
For comparison, the operating temperature of the Kestrel units:
14° F to 131° F | -10 °C to 55 °C Measurements may be taken beyond the limits of the operational temperature range of the display and batteries by maintaining the unit within the operational range and exposing it
to the more extreme environment for the minimum time necessary to take reading.
http://www.nkhome.com/pdfs/Kestrel_specs.pdf
That is quite a significant difference between the two. I only looked up the operating temperature when I started finding my iphone shut down for no know reason - when it powered back on it had battery left, and I wondered why - so I looked up the operating range, and was very surprised at such a narrow operating temperature. Now sure you can keep it inside your shorts or whatever to keep it warm - but I would rather go with something designed to operate in more of a temperature range where I operate.
But, make your own decisions. I am just presenting my experiences.