While this might only help a few...I have a Volvo iPod hookup in my car, and it works with my iPhone, but a screen titled "Accessory Attached" appears. In this mode, I am not able to navigate my iPhone's music. Instead, I am limited to the first 10 playlists on my phone.
Holding the Home button for six seconds releases the iPhone from the car's clutches, and I can now navigate through (while stopped) and play any song I want on my iPhone through my car's speakers.
I hope this trick not only helps other Volvo owners, but possibly others who have devices that are locked up on playback by their accessories.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070720034241570