Extend battery life with a power-controlling microcontroller?
Switching off power to the board only makes sense if you're using a full Arduino. Once you strip the Arduino down to the bare MCU and decoupling capacitors (or start with a basic breakout board instead), disabling unused peripherals and sleeping the CPU has much more of an effect since you no longer need to supply power to an external monitor chip.

Naturally this may involve creating your own board for the MCU. Fortunately Atmel has the basics of that covered.