If your tiny 3G iPhone USB power supply doesn't have a green dot, you need to go to this web site to order a replacement: Apple - Support - Apple Ultracompact USB Power Adapter Exchange Program.
It takes just a minute, you need to know your Apple store account and you need your phone to look up the serial number.
Apple will send a replacement - same size and form, but with a green dot. We're supposed to return the defective unit, rather than, say, keep it and have two.
Tough call, but honesty will doubtless force me to return mine in the prepaid mailer.