This suggestion may not be suitable for your project, but it's an easier alternative for low power and low voltage operation:
"Mini FET shield" http://www.sparkfun.com/products/9627
This board takes any input from 0.3V to 5.5V and outputs 3.3V, which is sufficient to run a standard 8MHz Arduino such as this one:
"3.3V Pro Mini" http://www.sparkfun.com/products/9220
To save power you can put the Arduino into sleep mode. The easiest way to do this is with the Narcoleptic
. This provides a low-power delay function which can sleep the Arduino for any desired time.
With this combination you can attain a total current consumption well below 1mA. Cutting the track from the voltage regulator's output on the ProMini (also disabling the power LED) can save even more power.
Running at 3.3V saves the bother of running at a non-standard CPU frequency, which would cause problems with sketch uploads, serial communication, delay timing etc.