Since my laptops see a lot of traveling, they get bashed around a lot. My two Vaios (one was the cute TZ) lasted about 1.5 years. I got a Fujitsu Lifebook P7120 after that (Pentium M, beautiful bright and sharp 10in display, great keyboard, no fan, lots of battery, robust casing) and wrote lots of presentations and a couple of books on it. Meanwhile, I also got a semi-portable MBP and would refresh it at every second revision or so. They lost about a third of their value, so the upgrade is really only third of the price. I also got the developer connection discounts, which help a little.
Meanwhile, I kept the Fujitsu as my portable alternative, because Apple did not produce anything satisfactory. In fact, nobody did. I did not want a 13in again. I ditched the MBA Rev A soon, because it sucked. I wanted to have something robust, silent and reasonably fast. Since I have the 11in MBA, my trusty old Fujitsu stays in the drawer. I expect to upgrade the MBA as soon as possible (i.e., perhaps in March or in July, when Apple switches to a core-ix solution).
I do not consider myself rich, but I use my computers as a tool to earn money. It is a simple calculation. Waiting time and frustration time (and sometimes time where I left my laptop at home because I could not be bothered to lug the weight) are times where I do not make progress on my projects. This quickly adds up to a few hundred Euros. With everyone that earns money using computers, I believe. If you do the math, it is cheaper to upgrade regularly.