I see a lot of people use Lipo batteries for projects and rely solely on the Arduino ADC to monitor battery life (which isn't very accurate).
You might want to check out the below thread. Coding Badly helped me to come up with a method where one can accurately measure the voltage powering the Vcc pins on the processor chip. It utilizes the internal bandgap reference and does not require any external wiring or voltage dividers. Should be good for measuring the state of charge based on a li-po 4.2 to 3 volt discharge ramp.
By using this correct Vcc voltage value one can remap the A/D conversion such that A/D will maintain constant calibration even with a slowly falling battery voltage.http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1294478456
While not as sophisticated as the method used with that I2C chip, it is simple and cost effective in just determining when the li-po voltage is getting too low and take action like auto shut off, alarm, etc.