Zap! released a press release announcing that their iZap for iPod external battery works for the iPhone and will triple the charge of the iPhone. Aside from powering the device, iZap can also be used for recharging the iPhone's internal battery.
"The iZAP and iPhone are compatible right out of the box, almost like they were made for one another. You don't need to buy additional connectors or cords and with the iZAP cradle you can recharge your iPhone right on your belt."
The external battery costs $79.00.