I'd like to use an Arduino Yún in a somewhat outdoor environment (in a metal box on a truck). I think I've seen that the temperature requirements of the Uno are -40 °C to +85 °C, but I can't find that information about the Yún. It would be nice to know where to find information about the temperature and humidity safe working requirements for every Arduino model.
The AtMega32u4 (the chip that drives the Arduino Yun) has an operating requirement of -40 to +85, so if you go outside that range, you will definitely void warranty (which is not to say it will break, of course!). See datasheet at http://www.atmel.com/devices/atmega32u4.aspx
A datasheet for the AR9331 seems to say that it only works down to 0°C ( http://www.eeboard.com/wp-content/uploads/downloads/2013/08/AR9331.pdf page 305, "Commercial Case Temperature") - this seems awfully high for a minimum temperature.
While that does not mean that, inside that range, you are safe, I would GUESS that you probably are - I would guess that the AtMega32u4 or AR9331 is going to be the most delicate part in there.
Of course, to get a definitive answer, you (or someone) would have to check the specs on each part.