The Thinkpad (I only own couple of very old old ones) has a semi-plastic case, but has quite a few special features to ruggedise it:

(Quoting from https://secure.wikimedia.org/wikipedia/en/wiki/ThinkPad#Leno...)

The following are some of the changes that have been made to the ThinkPad line:

    Added Magnesium-alloy chassis roll cage to reduce motherboard flex caused by holding the laptop one handed on a corner.

    Added Magnesium-alloy lid roll cage for a sturdier lid while replacing the lid material from magnesium-alloy to plastic for better wireless signal reception.[12]

    Added Carbon Fiber Reinforced Plastic to 15 in (380 mm) ThinkPad Models.*