I tried to power the arduino with less than 5 volts (3.7 to be exact) via the VIN pin, and everything seems to work properly.
Obviously if I had to interface the board with external logic (expecting 5 volts) it would require 5 volts power source.
But considering that the micro itself can be powered at 3.7 volts (even less), and that my circuit (a simple light trigger) can work at 3.7, can you foresee any issue?
On the "hardware" page it says it could be unstable...but why if the micro is fine at 3.7?
You may ask why I like 3.7V so much...well...that's because I'd like to recicle a nice LiIon battery;)