The iPad 2 has only just been released in the US but intrepid individuals are already at work integrating it into their cars. "I got Facetime in my dash!" Dude.This video from Soundman Car Audio goes through the process of installing the iPad 2 into a Ford F150 dashboard.
Besides having the know-how to work with fibreglass to develop the enclosure, they use an adaptor from Scosche to plug the iPad into your system and get it charging.
Ideally you can connect the audio via Bluetooth, but you should also be able to get it into your speaker system using the line out from the cable.
Facetime from your car is awesome -- though not something you should do while actually driving -- and if you are driving around without Wi-Fi access you will either need to jailbreak to get Facetime working over 3G or use a wireless hotspot.
There are a ton of useful applications for this integration, including music and turn by turn directions.