I just saw this, maybe this helps:
The iPhone can now play music through many previously incompatible car adapters and other external speakers originally designed for the iPod. In the first software build, the phone wasn’t recognized as a music source by some of these devices, which get the music through the iPod connector on the bottom rather than the headphone jack. For instance, after I did the update, I was suddenly able to route the iPhone’s music output through my car iPod adapter, which the iPhone had previously recognized only as a charger, even though it acted as both a charger and an audio adapter for my iPod.
The article where it came from is here:
http://mossblog.allthingsd.com/20070...hone-features/