Got a touchscreen head-unit in your car? Is it 9.7-inches and running the iPhone OS? If not, then I'm afraid you're not as awesome as SoundMan Car Audio, who celebrated the launch of the iPad by crafting a custom mount for the tablet in the dashboard of their Toyota Tacoma truck.
Video demo after the cut
When docked, the iPad is the sole A/V source for the whole vehicle. An Onyko ND-S1 is used to bridge the Apple tablet with an Audison Bit One which acts as the D/A convertor and processes the audio; that's then hooked up to a McIntosh MCC406M 6-channel amplifier.
To be honest, this won't suit most car interiors: you'll need a big space in your dash in order to accommodate the iPad. It'd also probably be more useful with the upcoming WiFi+3G iPad, which will add in a proper GPS receiver and turn the tablet into a huge PND.