Some USB ports do not provide enough power to charge the 3G such as some USB hubs. You plug it in and you see the "Plugged In" icon on top of the iPhone but it's not charging...no charging indicator...instead it depletes the battery quicker leaving it plugged in this way. I noticed this happens when I plug in into my USB hub from a laptop cooler. Plugging the iPhone directly to the computer charges it just fine. So becareful when using your USB device to charge a 3G.