Looking for consistent battery life for your iPhone. 3G? Check out this video and you can see a new “3-in-1 External Rechargeable Battery for iPhone 3G (Rechargeable battery, Mobile Power and Protective case)”.
It comes with 3 pieces: base shell, top shell and the rechargeable battery. It looks like it keeps the style of the iPhone in tact, but measures in 128 x 66 x 20mm. No word on the weight but my thoughts are it would be hefty.
A good point to this product is that it allows you to sync with your Mac or PC while the external battery is in use, and also, you can schedule when to charge your iPhone 3G. This will set you back $43.99 (US).