I haven't looked very closely at the iPad but on the surface it seems the dock connector can be used to provide power and usb ports (since they have an adapter to connect a printer). As long as the CPU is up to the task, and you can work within the disk space constraints, once it is jailbroken I think it's all you need. The GPS and 3g can be provided over wifi by a jailbroken iPhone. You hard mount the iPad and you just need your phone with you when you are in the car. We can write software on both sides to meet our needs.
As Bugbyte says the key is we have an affordable touch screen/computer combo now - that's what was missing before.
If USB ports can be added/accessed via the dock port, then you can add exta disk space, GPS etc directly.