I'm using my arduino in a sealed environment to monitor heat and humidity. Currently it's powered by a 12v 500mA wall-wart and the little guy gets a bit warm, not hot, but warm enough to skew the conditions within the environment.
Would using a lower voltage power supply, say 7v if I can find one, enable the arduino to run cooler?
The only other caveat to my system is the arduino shares the power supply with a little humidifier that turns on when humidity drops below a certain point. The humidifier is very simple and comprises of a small computer fan and a sensor to tell it when to run. The humidifier came with a 12v 200mA supply. Would a 7v unit deliver enough power for the arduino (while letting it run cooler) and humidifer?
I'm only going by what I read in the Arduino specs (http://arduino.cc/en/Main/ArduinoBoardDuemilanove) but it says there that 12V and above is indeed likely to produce some heat. It also says anything below 7V could be a bit too low for the Arduino and could cause voltage to drop below an appropriate thresholds. So you could consider going with somewhere around 9V maybe?
Obviously this does not answer your question about the fan... I think that depends largely on the fan itself.
The heat comes from the voltage regulator needing to dissipate excess voltage. Current is pulled, not pushed. So, the extra current capability of the power supply will not make a difference in the temperature.