An app which use the accelamometer to detemine if the G1 has been sitting still for a period of time - user configurable. If it still, it shuts off services such as wifi, G3, bluetooth, gps to save battery. My gps logger does this. Different from locale, because if I set it down on my desk for a while, it would basically shut a bunch of stuff off. Upon motion it restores to the prior state. Or upon use - again a user selectable option. So if I take it off my desk and stick it in my pocket it would stay in the "low power" state if I chose that option. And if it was plugged in I could choose if it would fall to the lower power state. Kind of like the power settings on a laptop.