It was my understanding that this is one reason why Apple does some strange accounting and why there are fees to get your iPod Touch upgraded, but not your iPhone. Part of the price of the phone is set aside to pay for future updates. So, you're not just paying for an iPhone, but also for access to future versions of iOS (up to a certain point). So not only is there an App Store incentive to get your to upgrade your iPhone, you already paid them to do it.

It doesn't seem like other manufacturers do the same thing. This could explain part of why they take so long in upgrading Android versions.