Sony Z series --> Carbon fiber
Dell Precision M --> Magnesium
Thinkpads --------> ABS with titanium under it, liquid drain etc.
HP Elitebook------->Magnesium/aluminium body.
They all can take a fall or two without a problem. Have really tough hinges and great keyboards without the slightest flex or noise.
Construction wise the unibody is not as durable as something made of leafs of alloys since aluminum is a very soft metal. It does dissipate heat better though. It has no kind of protection against liquids and the glass in front of the screen simply shouldn't be there.
The mistake people do is compare apple's stuff with other manufacturers entry level laptops. Look at thinkpads, elitebooks, precision M's, old Portege's (toshiba has really gone downhill) and the Z series and they are all comparable or more expensive than macbooks.