The gap from XP to Vista was because Microsoft screwed the pooch merrily with Longhorn, effectively cancelling the entire project. Microsoft actually historically have similar upgrade frequencies to Apple, especially now that Apple is getting to 2 or 3 years between upgrades
now that Mac OS X is a far more stable, evolved beast.
Never mind people getting through a Windows laptop every two years before upgrading. Not that I doubt that there are loads of people doing the same with Apple laptops, but the reason for upgrading their is often different

Again, single datapoints mean nothing I guess, but historically Macs have been known for longevity, and my 4 year old iBook doesn't disprove that in my mind, subjectively.