Well his a Juice Pack for the iPhone 3G. This new gadget for iPhone has been released by Mophie , and this gadget offers instantaneous power as and where you need it. Just make sure the Juice Pack istself is fully charged beforhand.
The iPhone 3G Juice Pack is touted to offer up to 350 hours of standby time and half a dozen hours of 3G talk time. This will cost you 99.95 $.