pito: ..and what is interesting for a low power design is the system can measure its battery voltage in the way it will not consume much energy. So you need an op-amp (sourced from i/o pin or switched on/off when required) with fet intputs, and a high impedance voltage divider (not to discharge the battery too much). I am using MAX1724 3V3 stepup (great chip -sot23-5)and my system is powered from one nimh cell.. From time to time it measures the battery voltage.. The 1.2V cell is charged from a sollar cell.. It works from 0.6V (it shall work from 0.8V input voltage according to the datasheet).. P.
If the processor is being powered directly from the battery voltage, there is a way to directly read the Vcc voltage of the processor without any external components. It's done by selecting the internal bandgap voltage as a analog input measurement (not as a reference) and calculating what the Vcc actual is. Here is a link to the topic as Coding Badly and I were playing with the function. http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1294478456/8
However as you are using a step-up regulator that would not apply as the vcc should be constant until battery exhaustion.