There's a hundred Thinkpads on the ISS right now, and zero MacBooks. There's a data point.

And if you're concerned about useful life, then I can tell you right now that you'll get major OS updates for about three years. Windows 8 requires basically a good graphics card, 2 GB of RAM, a dozen GB of space and a 1 GHz CPU. It'll run on your 2005 Celeron as well as Vista did. May not have been well, but a 2006 PC will, on average, run Windows 7 well enough.

Meanwhile, in Appleland, the Mac you bought in February '09 won't run the next version of OS X that's just about to come out. Seriously.