Extend battery life with a power-controlling microcontroller? This article describes a method to extend the battery life: use a low-power controller to switch on the Arduino only when needed. The author states that this method uses considerably less power than using Arduino's sleep mode.

Are there downsides to this approach? Which variant should I build?(The TPS61240 variant seems to use something from Circuits@Home that's not available anymore.) I'd appreciate schematics or links to detailed instructions.

Background: I'm building a sensor device that's in an environment with no mains power supply. The sensor should wake up eg. hourly, read eg. a temperature sensor, possibly send an SMS and then go back to sleep.
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.