At this year's CES, Visteon brought forth their iPad dash product which not only integrates into an auto's audio controls, but it has the ability to handle all interior electronic controls. The iPad communicates via Bluetooth with the box pictured to the right of the iPad, above. That device is installed in the car, in place of controls for climate, seat positioning, and engine information, among other things. There's also weather information displayed within the interface, access to a GPS, and more.
There is a dock connector for the iPad, at the bottom of the iPad holder, but it's to provide power only. Obviously you're going to need a 3G-enabled device and a data plan to go along with it, in order to take advantage of the weather and GPS.
It's not clear what cars this device is planned to be compatible with, but from the placement and size of the iPad holder, I have a feeling compatibility will be limited.
Engadget also has a nifty hands-on video of some of the iPad app's functionality.