I have been wondering the same, it appears that some people are saying that the new iPhone/iPod USB adaptors are only 5V whereas the old firewire ones are 10/12V
i have also read a forum saying someone tryed to use the iphone with firewire, and it caused the phone to overheat and DIE!!!
but i am very keen to do this my self with the old firewire/usb cable, so i can sync with USB and charge with firewire (no swapping arround cables)
hope this helps~