Apple has announced the adapter that came with your 3G may be defective and will be recalling them. See http://www.apple.com/support/usbadap...changeprogram/
for details. New adapters will apparently be available on 10 Oct free of charge.
Apple recommends you stop using your original adapter and charge the phone with the USB cable and your computer. I don't know if that's really necessary of if Apple's lawyers just got loose in the boardroom. Mind you, it can't hurt to err on the safe side.