i have the same problem. i'm a bike messenger in montreal, quebec, and if i keep my n800 in my bag (wrapped in a hanky inside an insulated camera case) when it's below about -10c, it often won't turn on when i get inside. if i keep it in my inside jacket, no problem, so it seems like external moisture (mild persperation) isn't a problem, but temperature-change-related condensation is. i have never seen visible moisture inside the battery area, but i've seen a bit on the screen and outside back cover.

to deal with the problem, i plug in the charger, and it will power on after about 5 minutes. even if the battery was fully-charged before i went out, getting cold seems to drain it completely. i don't know if the temperature is messing with the battery or the charge regulator, but just in case, i usually power it off again, unplug it, remove and reinsert the battery, and then plug it in and turn it on. even then, the battery meter still indicates a very low charge. (i haven't installed the command-line battery util.) this process is very annoying if i just need to look at a map.