Both factors matter: mobile systems are exposed to more abuse (and are generally constructed more, well, tightly); therefore, build quality matters more than in a spacious, stationary tower. And most of us would have difficulty repairing notebooks, so the quality of vendor/manufacturer support is more critical too.
I think everyone who really carries their notebook around has dropped it and/or dropped something on it. I don't often take my MacBook Pro outside (it's a tad too bulky for casual mobility, plus I'm a bit overprotective of it), but I've still dropped it, albeit from a very low height.
Other than impact and spillage, vulnerabilities are often to do with heat and moving parts (drives, fans, possibly
dodgy nVidia graphics, heh). You may want to look for models whose innards are easy to access.
Go to a consumer electronics shop and closely examine the build of a 'unibody' MacBook Pro. Then take a look at a typical Windows notebook. Which do you think will take abuse better?
I have the last-gen
pre-unibody MacBook Pro, so it's getting towards 2 years, and looks nearly mint, but as I said, it hasn't really been out in the real world. My buddy who works for Apple rode his MacBook Pro pretty hard, and he had to replace the hard drive at about 2.5 years. My ex-girlfriend's white MacBook is almost 4 years old now, was her primary machine at home and uni, and it's starting to get somewhat wonky now. I expect a unibody MacBook would be much more robust than these.
As Emulex suggested, if you get a Windows notebook, try for one from a 'business' line. They tend to be better constructed (magnesium frames, etc) and sometimes offer better support. ThinkPad is the standard, but everyone seems to agree build quality has suffered over the years.
Get a good warranty.