Thanks to this great work from Adafruit coming from Limor I was able to make a simple circuit to let my 2nd Gen iPod Touch (running the latest iOS 4.1) charge from any USB source, not only from PC or from original Apple chargers, but also from any USB charger.
Here is the schematics I've used:
To match the resistor values I've used a couple of 1% resistors to get the nearest value I could (with the values I had).
Here is a picture of the extension cord I've made, together with the small stripboard to host the voltage dividers.
The voltage measured across pins 3 and 4 and ground is very near to 2V for each pin. The extension cord allows to adapt any USB charger to work with my iPod Touch 2nd Gen (with iOS 4.1).
I guess the same works also for iPhone.