"Charging is not supported with this accessory."
That is the message I get on my iPhone 3G when I plug it into:
Custom iPod interface in my Mazda Miata
Factory iPod interface in my Infiniti EX35
Factory iPod connector included with my Pioneer Elite A/V receiver
Bult-in iPod connector on my Logitech iPod speaker system
All of those devices use the 12 volt Firewire pin for charging.
Happily, I never used my previous iPhone with these devices for musical playback. I've got iPods for that. But I DID like the idea that I could plug my previous iPhone into either of the cars and recharge it while driving. (I never needed to do it, but I liked the idea of it just the same.)
Now, with the iPhone 3G's use of power-sucking 3G and GPS, the likelihood of needing a recharge mid-day has increased. So now I'll be sure to carry a USB-based cigarette lighter adapter and a short USB iPod cable in the car. A bit more of a pain, but it will get it recharged as necessary.
BTW, SOME home-based iPod Audio systems (speakers, receivers, etc.) use the 4 volt USB charging pin. Those will charge an iPhone 3G just fine.
IMHO, removing the 12v Firewire charging connection in the iPhone 3G was a major boneheaded move on Apple's part! I'm sure they had their reasons (like, to encourage the continued sale of regular iPods!) but it was still a stupid decision!
Mark
Edit: Apple covers the topic (briefly) in this KB article:
http://support.apple.com/kb/HT1476
It is also mentioned in the iPhone 3G user manual, page 34.