I can confirm altitude isn't a problem, I've taken mine to over 17000 feet without any problems. Can confirm screen slows down a bit, but this is due to the cold. Carry spare batteries, they go pretty quick. If you feel it's getting too cold, you can always put it into an inside pocket, your body heat should bring down the temperature drastically (also applies to spare batteries, they will retain more juice if they are warmer). Overclocking also brings down temperature (I love winter with the N900). Carry your phone in a pastic bag to avoid humidity (if there's lots of snow).

Good luck.

And in those kinds of extreme circumstances, never rely solely on electronics to save you in case you get into danger. They usually tend to not work when you really need them :-)