For those of you who have iPhone 4 (or any other iPhones) and you want to make a portable USB charger, here’s the MintyBoost USB Charger Kit you can use from MakerShed.
Works with the new iPhone 4 and more! Build your own MintyBoost: a small and simple (but very powerful and very MAKE-like) USB charger for your iPod or other MP3 player, your camera, cellphone, and any other gadget you can plug into a USB port to charge. Mint tin not included. See more details tab for a list of compatible devices.