USB (5V) charging isn't new to the iPod. That change happened when Apple switched from including a Firewire cable to a USB cable. That was "ages" ago. Since then, Apple has provided BOTH the working 12v Firewire pin and the working 5v USB pin on their iPod/iPhone devices. That is, until iPhone 3G. The iPhone 3G has the 12v pin, it apparently isn't connected to anything inside. With no 12v charging circuit in the iPhone 3G, no firmware upgrade is going to fix it. It was apparently intentionally designed without the 12v charging circuit, probably to save on build costs (if Apple saves $1 per iPhone due to this, they've already saved or $1 million!).
Since virtually all car-based iPod connectors use the 12v Firewire pin to charge, it is almost certain that 99% of the car-based chargers/docks/connectors won't charge the iPhone 3G. (I know the factory iPod connector in my new Infiniti EX35 won't charge my iPhone 3G and it charged my original iPhone just fine.)
But what is amazing is how many home-based accessories also use the Firewire charging pin! You'd figure those would use the USB pin. Even my Pioneer Elite A/V receiver (which is less than 5 months old) charges via Firewire 12v over its included iPod cable.
Fortunately, I never used my original iPhone, nor will I use my iPhone 3G for these in-car situations. I've got 3 iPods for that. In fact, I simply leave an iPod in each car all of the time (except when I bring it in the house for a few minutes to update the content with iTunes).
But I DO think it was a stupid decision by Apple to remove the ability to charge via Firewire. I'd happily had paid an extra buck or two to keep that capability. Whatever bean counter made THAT decision needs to be given a swift kick in the ass!
Mark