This deal has ended.
Update: The price has gone up. For a limited time, Buy.com has the Cables Unlimited iPhone BatteryBoost on sale for just
$29.99 $33.01. About the same size as a standard protective case, this has a separate rechargeable battery that can extend the life of your iPhone 3G or 3GS two to three times. Budget shipping is free and there is no tax (except CA, MA, MD, NY, TN). Compare this with $32.52 at Amazon.