Isn’t there already a MacBook Air?
It just seems that trying to use the Mac UI purely by touch would be godawful. I mean how do you Command-click, or right-click a UI element with your finger? These are solvable, but only with horrible compromises. The iPad has a completely different interaction model to very good reasons, and this would utterly butcher it. Microsoft tried this approach and it was disastrous.
Yes the iPad has its limitations, but also incredible strengths. The new multitasking gesture system is a vast improvement.