A little while ago then I stumbled on this very good article: http://www.ladyada.net/make/mintyboost/icharge.html
In which they decribe how they produced a battery powered USB charger. After reading that article I took my cheap USB charger and decided to modify it.
This will be a really, really easy modification and I think that anybody with a soldering iron could do it.
Step 1: A little Theory
Nowadays the iPhone expects a certain voltage on those two pins to decide how much current to absorb from the charger. Putting a 2.0 V voltage on both the pins the iPhone will absorb about 500 mA, while with 2.8 V on D- and 2.0 V on D+ it will absorb about 1000 mA.
The same behaviour I expected to be observed on my iPod.
On the images there are the schemes for the two configurations. As you can see, using an opportune couple of resistors it is possible to get the voltage required. Obviously the 1000 mA configuration is better if you want your phone charged quicker, but it is possible that your power supply can not support that much current.