I'm planning a trip to the mountains I'll be at 15000 feet above sea level at temperatures as low as -20 f. Would it be okay to take my phone with me or should i take my other phone?

It should be OK, although the batteries will have less juice at that temperature.

i think it would be more a problem with the screen; but i am not really sure; afaik most screens are for temperatures not below 0 C (or around 33 F); maybe a look in the manual of the beast?

N900 will continue to operate in -20F (around -29C), iirc the minimum temp it can take is -45 according to the manual.

Overclock it to 2000MHz while you're there!

I have used my N900 at -33C last winter, I guess the temperature was even below that because of strong wind. Screen didn't update as fast when it was cold, but still very usable. Small 'ghost' effect was only thing I noticed.

Just keep in mind that when battery gets cold, it doesn't give power as much when warm and when you take your N900 back to warm, keep it in a backbag for a hour or something so condensation is minimal.

(Many times when I'm out in the winter, I just keep my N900 in my pocket, it's a bit warmer but lot more condensation...so far, no problems with that thought.. )

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 :-)