In the house here I have my 14" white MacBook and black 13" MacBook and both still look pretty good appearance-wise. I sold my 2009 MacBook Pro but am still using the 2012 one, and both look brand new.
From a durability point of view, the unibody design was the best thing to ever happen to portable Macs. A modicum of care and cleaning and they look pristine for ages, though this is of course contingent on you not dropping them!
I'd add, though I won't be popular for saying this, that Apple's drive to limit user-replaceable parts has made modern Macs much more trouble-free as well. I worked in a repair shop, and the broad rule of thumb was that the more "openable" a Mac was, the more of them we'd have in the back room for repair. The exception to this was what are sometimes called "road apples," ie models with real production issues (the corroding capacitors on certain eMac models springs to mind ...). Living in Florida at the time, the other exception was people who didn't get a UPS ... we'd see lots of them after every lightning storm or hurricane ...
I don't like sealed in Macs, but I have to admit that the move is driving repair shops out of business. Barring a lemon part or something like that, the non-upgradable Macs seem incredibly hardy compared to say 10 years ago. I should note in the interest of fairness that a friend of mine had a Toshiba notebook way back when that had been dropped, run over, just about anything you can think of and kept on ticking. There was at least one PC that didn't go down without a fight!
Now, as Nick says, if we're talking useful lifespan, that's another issue -- changes in general (non Apple) technology, changes in Internet technologies, and of course changes made by Apple software-wise all contribute to a definite shortening of the "useful" life of Macs compared to 10 years ago. It's nothing dramatic, but we think about active users either upgrade or replace a bit more often now on average than they used to, say every five years instead of six-seven.