What are the environmental requirements of an Arduino?
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.