I like Griffin stuff, so I was willing to take a gamble on the just released Griffin Firewire to USB Charge Converter for iPod and iPhone 3G.
In short, it works. One of the truly infuriating iPhone aggravations has been ameliorated.
The $20 device is a bit bigger than the proverbial postage stamp, about an inch square. It's quite light. The standard iPhone/iPod adapter is the modern friction-only style, no lock feature. (I'd prefer the positive lock for this use, but Apple has moved away from them.)
I plugged one end into my SONY car stereo with iPod adapter cable and the other end into my 3G iPhone. The iPhone began charging. More importantly, the kludgy but useful iPod control software works, and the sound quality really is better through that cable than through the AUX in mini-jack connector.
I did get an error message from my iPhone saying the device was not compatible with the iPhone and offering to reduce audio interference by putting my phone into airplane mode. This is not related to the Griffin converter, I get this message without it. It's different from the "can't charge" message -- it's saying that the radios built into the iPhone can cause interference with many devices not built for the 3G iPhone. In my case there's no hum or other problem in standard mode.
The adapter does make the cable to iPhone connection long and somewhat fragile. It would be easy to whack the end of this longish lever and injure a connector. It would be best used when the iPhone is securely mounted.
It's probably not worth buying this device to extend the utility of a firewire charger -- it costs almost as much as a Griffin USB charger and cable. It's really made for an automotive head unit, and it works on mine.
Note that Griffin makes no claims that the automotive head unit controls and audio inputs will work -- only that charging will work. I took a chance that everything would work, and it did.