3G (all CDMA really) requires a ton of power to transmit and receive data and voice. That's why GSM phones have such ridiculous battery life ratings compared to 3G phones, and why the blackjack ships with extra batteries and chargers (keep a battery at work and one at home and swap them twice a day). Shipping the iPhone as a GSM/EDGE device means it will get a lot more life out of a charge than if it was a 3G device.