The combination of lightness and rigidity you keep talking about is probably - ta daah - the titanium. The reason titanium is useful for high-tolerance uses like spaceships and fighter jets is because it's incredibly rigid. The disadvantage is that it's not like steel, which will flex before breaking. This makes titanium 'stronger', but instead of bending and buckling under strain like steel does it just snaps. The other disadvantage is that its rigidity makes it harder to manufacture. The first disadvantage - snapping instead of bending - would seem to make absolutely no difference in laptop construction, since any force or impact strong enough to snap the titanium is going to break the delicate electronics first anyway. The second disadvantage, the difficulty in manufacturing, makes the titanium housing another shiny, expensive apple luxury item.