Before the mac flaming starts, consider the possibilities:
iTunes, GPS, 9.5" display (that orients vertically or horizontally), software that's touch oriented, the App Store, Accelerometer, web browsing.
The big key to this will be "will the release OS allow multitasking?" We don't know. Additionally, how much freedom will we have with the dock? Can you connect cameras, external hard drives, other USB devices?
What we do know is that it will be able to run any app that can run on the iPhone. We also know that future versions of those apps will have iPad optimized designs.
So visualize this:
The screen is divided top/bottom, with the top running GPS through software like Navigon ($60), and the bottom running iTunes.
Through the docking station you input OBDII (USB) and run a secondary program that allows for customized digital gauges. Whatever the ECU knows, you can know. Boost, pressures, temps, whatever. Float those guys along the sides or top and have your glorious interface.
Want a new song? No problem, you have the iTunes store right there. Videos? TV Shows? It's really great.
Now, I run a nice setup in my car, but it had a price tag. Mac Mini 2.26 GHz ($800), 7" Lilliput 629NP ($230), USB GPS antenna ($90), Car-USB radio antenna ($180), Mounting brackets ($150), Power Adapter ($135), not to mention audio equipment.
Total up all that system specific stuff and you're will into the $1500 range for the system, AND I DON'T EVEN HAVE INTERNET.
Price point on the all-in-one iPad:
(I'm including the 3G option as you need the GPS and want the internet. If you don't, then just subtract $130 from each)
Relocate HVAC somewhere. Ditch center 2 vents (or just adapt the vents on the new bezel you'd make). Beautiful.