I built such a system with an Arduino a while ago using the bluetooth shield (a huge energy hog). They key was adding some basic "sleep when inactive" routines, it wasn't that creative, if I remember rightly it was about ten extra lines of code to have it sleep for a while if it detected it wasn't in use.

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!