Morphie Juice Pack Plus http://snapvoip.blogspot.com/
I have used the Morphie Juice Pack with my old iPhone 3G and it did supply enought juice for the phone while it worked, but suddenly if decided not to get charged up, I am sure something to do with the charger connection.
Now Morphie is coming out with Morphie Juice Pack Plus Outdoor edition with similar juice pack for iPhone 4 and comes with an app that helps you to get around, basically an GPS app (a collaboration with NeoTrecks) for the iPhone 4. There are many GPS apps in the iTunes store but you get this one for free and looks to be taking you where you want to go according to Engadget crew who seem to get the bearing of their office. You can read more about the duo and the see a video over there.