Arduino Fio External Charging Light

I just got an Arduino Fio from Sparkfun (awesome little board), and want to put it inside an enclosure. I would like a light on the outside of the enclosure to show when the Arduino is plugged in, and not on battery power anymore. I know there is the charging light I could tap into, but I want the light to stay on, even if it isn't charging anymore (the charging battery turns off when the battery is charged, I believe).

Run a line off of the USB input with a regulator to one of the arduino pins and monitor it? Is there an easier way?


I really just want the Arduino to be able to tell whether the Fio is plugged in to external power. Would I just regulate the external power input to 3.3v and then read that with a pin on the Arduino?