The hardware aspect isn't all that difficult, the challenge remains the software and interface. Is there any work on the Android side to make a more car friendly interface? I remember some would go into a car mode when docked.

Another hardware idea was funded on Kickstarter. This one goes the route of docking the phone right in the Dash, and has a faceplate that can be swapped to accommodate different phones. They are also working on the software side to provide a better car UI.

http://www.devium.com