On the surface, not much has changed. The iPhone 3G is slightly thicker, 0.48 inches to 0.46 inches to the older iPhone. However, the iPhone 3G also shed an imperceptible tenth of an ounce, to 4.7 grams. All of the other specifications remain the same, save for cosmetic improvements like a minijack that's flush with the chassis.

But there are some interesting changes on the inside: 10 hours of talk time while on 2G (but only 5 hours on 3G), compared to 8 hours for the old 2G iPhone; and 300 hours of standby time, compared to 250 hours.

While on a 3G network, Internet surfing time is rated at 5 hours (6 on Wi-Fi) versus 6 hours on the older iPhone. Video and audio playback times remain unchanged, at 7 and 24 hours, respectively.

At this point, it seems reasonable that a 3G radio should consume more power than a 2G radio, although I would think that the power consumption would also be affected by the time in which the radio is actively powered on, as well as the process technology in which the chip is fabricated.

I guess my question is: does AT&T allow customers to specify which network they operate on, and switch back ad forth easily? I'm not aware that they do. Using a 2G network, iPhone 3G users will see an improvement in battery life. But I suspect that the new iPhone 3G users are going to instinctively surf and talk using the 3G network, which is going to effectively halve the battery life versus the first-generation iPhone.

EDIT 12:02 AM EDT: (Thanks Justin!) Engadget points out that there is a 3G enable/disable feature: "The enable/disable 3G setting is real, and buried a few menus deep. There is no automatic switching, Apple just assumes you'll leave 3G on, and that the iPhone has the juice to support that usage."

Hmm. If I were a developer, I'd be racing to come up with a little applet that would turn on 3G functionality when I was surfing the Web (default behavior: turn on 3G when loading Web browser), and otherwise turn off 3G to maximize battery life. Colleague Matt Safford also wonders why the battery life is lower when surfing using Wi-Fi. Is that because the 3G radio is also on?