The main power consumer in the GSM case is of course the GSM transmitter, so it's no suprise that he can improve power consumption by turning it off.
Generally on more powerful machines you take for granted the operating system, firmware and their many conveniences, but often there will be bugs or poor design decisions that increase power consumption by quite a lot.
For those not interested in Contiki, late in the post there are some power consumption measurements that illustrate rather clearly why switching off the 60 mW radio as much as possible is a must as soon as you go battery.
You can find the Contiki-port at https://github.com/msloth/contiki-launchpad
Not sure what you mean here, a microcontroller platform like Arduino can be extremely low power. The AVR chip itself uses a few mW running at full speed, and less in sleep mode (less than an LED). What uses most power is other devices in your circuit, you need to be careful with them and shut them off when not needed.
The one gotcha was making sure the board being used had a reasonably efficient regulator on it, some of them have regulators which draw a (relatively) huge idle current so putting the chip to sleep has almost no impact.
Hope that's of some use!
http://hwstartup.wordpress.com/2013/03/11/how-to-run-an-ardu...
