I have been wanting to get better integration with my iPhone in my car. Normally I would just connect my iPhone to the line-in jack of my radio. The problem with this is that, if I want to skip songs, I need to wake the iPhone, enter the password, then select the skip track button. This is pretty dangerous while trying to drive. So I have been interested in getting a solution where I can use the controls on my head unit to control playback from the iPhone. In addition to controling the iPhone, I wanted a solution that would look stock in the car.
2point5 is a company that specialized in custom installs of iPhone and iPods. They took this experience and now sell docks that are custom made for various cars. In addition to the docks, they also bundle the needed adapters for the various manufacturers' radios.
I bought the spec.dock package HONFGV2IPKG. This package includes the USASpec PA15-HON2. This adapter can be connected to either radio's XM or CD changer interface. When connected to the XM interface, song meta data is shown on the radio's display. Unfortunately, the XM interface doesn't work with the Honda headunit that has navigation, so the CD changer adapter was used.
I like the way the dock fits into the car. It really looks good. Also the integration with the radio controls works very well. I would recommend the spec.dock. Here is an album with some photos of the install.