The touch interface is simple in concept. You'll have to correct me if I'm wrong because I have no experience with using them... but basically, it's an input device similar to a mouse or keyboard.
Going the route where a computer is involved, there is already a program that would do exactly as we want, we just have to have a touch monitor for it. Check out Veency
But, I don't want the hassle of dealing with a computer in a car with bootups and shutdowns, it's too much wasted time (and battery for us hybrid drivers).
The other optimal way would be through Bluetooth as already suggested. I've done some poking around for information on this.
I would say this is on the edge of where we are headed with a touch interface. Correct me if I'm completely wrong. Ubiq has been getting results connecting a bluetooth keyboard with his iphone. If we can maybe use similar BTnodes or whatever, maybe there's hope and it's not that far off. I think most of the work has already been done for us, we just need to combine all of these elements together.