iPhone 3G cannot be charged while 3GS is charged I have a car USB adapter and can charge my 3GS with it. However using the same cable and adapter my friend's 3G cannot be charged.

First thing that comes to my mind is a problem with the 3G unit however it works well elsewhere with the same cable (e.g. 3 different laptops, wall outlet and even a PS2's USB slot). 

So I think there is some mismatch between the unit and the car adapter. But I am not sure what I must consider when ordering a new car adapter for the 3G, for example do voltage consumptions vary between the 3G and 3GS? Anything else? Or is it just a random problem and I don't need to worry and just buy a new car adapter?
I had the same problem, i had 3GS and de car adapter i am using is from philips, my collegeau with a 3G couldn't charge. After buying a new car adapter, it solved the problem.