Our hardware hacking friends at Adafruit Industries have revserse engineered how Apple's battery charging tech works. PT says:
In this 7 minute video we explore the mysteries of Apple device charging. Usually device makers need to sign a confidentially agreement with Apple who want to say "works with iPhone / iPod" and never talk about how the insides work. If you don't put these secret resistors on the data lines to you get the dreaded "CHARGING IS NOT SUPPORTED WITH THIS ACCESSORY". We demonstrate how anyone can do this and make their own chargers that work with iPhone 4, 3Gs, etc.