I finally fixed my milliamp meter and did some measuring. I have not tackled enclosures yet due to the added costs and heat dissipation concerns, but wanted to discuss it anyway.
With the LCD off, the unit (prebuilt) uses 20ma. Each level of brightness adds 20ma so that it is ~80ma when full on.
So the guino creates ~1/4 watt of heat with the lcd off, and ~1 watt with it on. The lowest rated component is +70C (158F), so some consideration needs to be given to not trapping all that heat. I don't think it magically stops working at that temperature, just that you shouldn't plan on going above that.
Since we are going from 14 volts to 5, the regulator itself is dissapating .18 watts w/backlight off and .738 watts w/backlight on setting 3. The regulator is easily the largest generator of heat here, and the backlight is the largest user of the remaining power.
Simple strategies:
Consider not mounting the mpguino in the sun. I usually set it in front of the instrument panel where it gets a bit of shade usually and is in a "natural" reading location.
Crack car windows on hot days to reduce interior temperatures.
If adding an enclosure:
Drill plenty of ventilation holes on top and bottom.
Turn off the backlight during day use.
(advanced) Consider adding an 8 volt regulator (and a couple small capacitors) to the power line away where power is tapped, this will reduce the amount of heat created by the internal regulator to 1/3 of what it would be at 14 volts.
(advanced) Remove the lcd transistor and power the lcd backlight from the cars panel dimmer via an externally mounted
~270 ohm resistor. This is a pretty elegant approach actually, the resistor won't contribute to the in-case temperatures, the backlight will be "in synch" with the dashboard lights, the internal regulator will be limited to .18 watts, and it is dirt cheap and not hard to circuit bend onto an existing mpguino.
Note: When/if I can, I will add a temperature readout to the cpu monitor display (apparently the 168P I'm using has an internal temperature sensor), that should help folks determine the state of cooling.