Ace:
From what I have been able to determine, the low temperature limits arise due to the inability of an LCD type display screen to achieve the required levels of display brilliance at temperatures below -20°C. This temperature seems to be the "industry standard" low temperature operational limit for any avionics display system that uses an LCD type screen.
That doesn't mean that you can't read the screen at lower temperatures, it just means that the screen doesn't provide the levels of brightness and contrast needed to meet the certification specs when it is cold-soaked below that temperature. This is not a problem for aircraft that only use these type of displays for navigation purposes (e.g. Garmin 430, 530), but it can be a problem for aircraft that use these types of screens for display of engine instrument indications.
Fortunately, the screen is very thin (only a couple of millimetres), and it is only necessary to heat up the screen itself - not the whole box - to bring the brightness back up to spec. There are different ways of doing this. The most high-tech way is to specify a screen that has a metallic coating on it that can be electrically heated up - Barco, a major vendor of large format aircraft display screens, makes such a screen. A lower-tech way is to just aim a hairdryer at the screen from about 18 inches away and warm it up that way. An even lower tech way is to wrap a hot water bottle in a towel and place it over the face of the screen for a couple of minutes.
Once the screen temperature is at or above -20°C, you can start up the aircraft and use the aircraft heating system to finish warming the interior up. Once the flight compartment has warmed up a bit and you have confirmed that screen brightness is satisfactory, off you go.
To be honest, I could not imagine actually taking off with a flight compartment interior temperature below -20°C. I think that even in the worst possible environmental conditions (e.g. -50°C ambient), one would want to get the flight compartment air temperature up to at least 0 before takeoff.
Hope this helps.