Anyone with the car and owners manual, could you check to see if it says how much power the USB cable provides? I'm expecting it to be 500mA or 1A. If its only 500mA it might not be able to charge an iPhone while its in use.
Can't (currently) comment on how many mA the USB port provides but the other evening I connected my Ipod Touch (with only 15% charge) and hit the shuffle button.
After an hour or so of driving (and listening to the maddest ecclectic mix) the Ipod was fully charged. Hope this helps.