MacOS X has a touch device - either the trackpad, or the magic mouse. So the OS can handle it easily. It's the users who can't handle it long term, because the human arms are not built for that. You can do a simple experiment: Open your laptop, then point your finger at each of the four corners in sequence. Repeat this until you think your arm is falling off. Tell us how long you could do it.
"touch" on a trackpad is very different from direct manipulation with you fingers on screen. the touch pad is basically a turned around mouse with your finger being the "mouseball". well not quite that. but basically.
I do a gree that a touch-only solution would suck on a notebook, however there are situations where being able to manipulate directly on the screen comes in handy. I think it will come one day on a macbook. But only as an option. Like quickly zooming into something via a pinch gesture or quickly swapping through albums or something like that.