Originally Posted by Beaverboy
I'm pretty sure the OP meant it the other way around... using the iPad to control the ICE.
Then everything would rely on having a connection unless you just want the content you brought in the car with you. This just brings us right back to the CarPC thing but with an Apple device. The need for amplification and audio processing is important, I highly doubt the sound quality from the iPad is going to be astounding, nor will it have enough to drive an amp on it's own.
There are tablet PCs now that could accomplish what the OP wants to do with the iPad.