Turning the entire restpad for the palms i.e. the space from the front of the laptop up to the space bar row would be theoretically pretty easy. The glass section of an iTouch/iPhone is pretty sturdy and thin, so it'd replace pretty well the palm rest area. It'd sure give the touchpad area a kick up the ass for a start (going from say 1.5' x 2' to 3'x 15' size trackpad, with multitouch etc). Presumably, if the sensor can sense things on the pad, upgrades could be software based, rather than firmware, so as things move on, you could add new movements (touch, pinch, drag, select etc) and the processing of the input could be put onto the CPU. The ability to use a touchpad in the same flicking style as a iPhone through coverflow would be cool.
There seem to be several Apple technologies in "sleeper" mode right now, and hopefully we'll see some come to fruition in 2008 (to use all those new Intel chips perhaps).