The logical and practical way to make something like this would be to build a laptop with a docking slot where the pad would normally go. Slot in your iPhone/iTouch and it docks, syncs, gets charged, acts as the trackpad/button(s) and maybe even as a security key (no iTouch, no workee). An advantage over typical trackpads would be the ability to display... whatever, some buttons or sliders for special purposes. If the phone is used, the audio would be channelled to the laptop sound connections.
I'd like a similar deal for my car audio too. Slot in the iPhone/iTouch and use it as the store for the music, radio channel choices, nav. software, car service record, etc.