For this project I've stolen a charging circuit from an Emergency iPod charger I got off ebay. You can find these all over the place. The key is to find one that will work with an iPhone.
Apple decided to have it's newer iDevices not follow USB standards. When an iDevice is plugged in, it checks the data tabs on the USB to see what it's plugged into. Depending on what it finds, it sucks more or less power, which makes sense but is annoying because NOTHING ELSE DOES THIS. Thus no charger out there has any power flowing to the data tabs.
So the key is to find one that works for your newer iPod or iPhone. If you have an older iPod or iPhone when you don't really need to worry all that much.
(For a USB Version of this, check out this instructable.)
If you want to make your own circuit you can always use a Mintyboost kit.