True, thanks, I came to realise that later(Original post by SMed)
I think there's some confusion here. There are two ways of getting audio from the 30pin connection at the bottom. You can get digital out (by USB) or analog line-out (via 30pin to 3.5mm jack). The USB output will not be DAC'd nor amped by the iDevice. However, if you use a 30pin-to-3.5mm line out plug, then you'll be using the iDevices DAC, but it won't be amped (so you'll use ONLY the head-units amplification).
It's much more convenient to just use the 30pin USB cable from the iDevice, as it will charge the device, display track information on the headunit and usually allow control from the headunit itself. In an environment like that, you won't hear any audible difference between using USB line-out or analog line-out.
If you can't do that though, the line out should suffice offering just the DAC so the cars amp can power it.
Chances are though, if its recent, it might have iPod connectivity so connecting by USB might be possible.