I have intermittent power-on problems with my new N800. I followed the other threads discussing various power-on problems but I could not find any common symptoms with my case.
My N800 does not power-on the first time in the morning when it is still cold in the house (I turn the heater off during the night). I usually fix the problem removing the back cover and reinstalling it. I thought the problem was the back cover but other users of this forum explained me this cannot be the case. The last time it happened again, I refrained from removing the cover, I waited 10 minutes for the temperature in the house to go up and the N800 powered on without any problem. When it is cold, I also noted condensation on the back cover just from the act of keeping the N800 with my warm hand.
Is it possible my power-on problem is caused by the cold temperature or the condensation?
Do other N800 users have similar problems with the cold temperatures?

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.

Condensation of any kind is definitely bad for electronics. It's easy to get shorts, particularly when there's some dirt or other pollution. It's a good idea to try to keep the device somewhat warm, at least avoid freezing conditions (I keep my N800 in the inside pocket of my jacket, so it's always at a good temperature in any weather).

It sounds a bit strange though that it should get cold enough for condensation inside a house just because the heating is off at night.. but maybe the air is very humid too? Obviously, if you see actual condensation there's a real problem, and you should be careful.