The two concepts at play here are planned and perceived obsolescence. The former is defined as creating items that are designed to malfunction after a certain period of time, forcing the consumer to make another purchase. The second is the idea that the producer will create new models or options that initiate dissatisfaction in the user who already owns an older model or version of the producer's product(s).
I'm a big fan of Apple, as most regular readers will know. But if I have an issue with the company, it's how proficient they are with perceived obsolescence. Their product design, marketing, and lifecycle have perfected the environment where many people are unsatisfied with their currently owned device after a period of 18-24 months (no science or stats to back this up, it's my own guess). Apple plays on this and, as Khoi says, it's good business. I simply happen to think it's morally the wrong direction.
If Apple wants an environmentally conscious person like me to take their green efforts seriously, they would encourage users to use the same device for years, rather than simply make recyclable, chemical-free computers. In my utopian vision, Apple still has plenty of market share remaining to keep growing. They could continue to build insanely great hardware products, ones that last for a decade, and amazing software that makes those devices a pleasure to use. Then they grow by converting those who do not own Apple products into new customers.
Of course, the real issue is not Apple. It's you. It's me. It's our culture — Apple is only one company taking advantage of our desire for more. I'm writing this entire piece as an owner of an iPhone 4S, an iPad 3 and a spring 2012 Macbook Air.
I can't really speak to whether Apple designs their products to purposefully not last, which seems to be Khoi's main point. I sadly never keep the devices around long enough to find out.