The A/V connection seems to be a valid reason, but...

Ever used a Ford with MS Sync? I rented a Focus with this on vacation last summer, and it works quite well - a surprisingly innovative and compatibile product for Microsoft.

It hooks up to your iPod/iPhone with the standard USB cable, and can play audio, so it has to stream it over the USB connection somehow, or it rewires the port, which seems unlikely to me, as you'd run out of pins (stereo audio, power, control, ground = 5 pins minimum).

I really wonder how this is accomplished, as it can play back protected AAC music, which must be decoded on the phone, so it isn't just using the device as mass storage...