Users are reporting erratic behavior of the iPhone 3Gs battery meter, specifically an issue in which the battery indicator reports a full charge and then suddenly drops unexpectedly along with a warning message indicating that the phone should be charged soon. Usually the meter at the upper right of the iPhones screen does not match the large red or green battery indicator displayed with the low battery warning or when charging.
In-house, we've noted discrepancies in the green battery meter while charging the iPhone 3G. Usually the green battery meter shows a great remaining battery capacity than the smaller one at the top right of the screen. Usually when this happens the iPhone also takes less time to charge so it's likely that the green battery meter is more accurate.
Users in Apple's Discussion Forums recommend rebooting the iPhone to resolve the problem. This is accomplished by holding the Home Button along with the Sleep/Wake button. Keep holding these keys ignoring the slider to turn the phone off. The screen clears release both buttons when the Apple logo appears.