I was thinking about this the other day after seeing the writeup on the main modmyi page about the electrical engineer that figured out the resistances needed in the data wires for the usb connection in order for the iphone to start charging.

Using this information, it's easy to make any power source a charging station for your iphone, right? Cuz my idea was to use a camcorder battery as a backup power source for my phone when I'm on a long bus or plane ride. They have batteries on amazon and ebay that are 6600 mAh for less than $20.