The iPad 2 has just been released, and the intrepid individuals at Soundman Car Audio detail how to install the iPad 2 into a Ford F150 dashboard. "I got Facetime in my dash!"
Besides having the know-how to work with fiberglass to develop the enclosure, they use an adapter 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.
There are a ton of useful applications for this integration, including music and turn-by-turn directions, and the basic idea would work with any tablet. Standard warnings apply: Don't drive and Facetime.