The central brain of our project is a DC to USB converter circuit. This takes our AA power and changes it into the 5Vs we need for charging USB. There are several ways of doing this.
1) You can make one yourself using Lady Ada's Minty Boost kit. It's $20 and requires soldering. It also charges almost every gadget under the sun, including new Apple Products.
2) You can buy a premade circuit off ebay, or even off my website BrownDogGadgets.com. They cost around $10 and work with most everything. Kindles, iPhones, iPods, GPS, Android Phones. They're great.
3) You can rip one out of a cheap USB charger. eBay and or Amazon are great places to look. Some of them don't work with Apple stuff, so be careful or buy several.
Whatever you do, don't try and make a 6V or 9V circuit and then use a voltage limiter to take it back down to 5V. That's sloppy and inefficient. You can do it, and it works, but there are far better options.