My 2007 Blackbook was still running perfectly when I upgraded to a 2011 MBA. The MBA is a massive step up from that MB but I could see myself still happily using the Blackbook had I never upgraded. I'm a little annoyed that both of my Macs have been stuck with Intel integrated graphics though....
Man, I would be happy if my 2011 MBA 13" (1,7 Ghz i5, 4 GB RAM) would be only "abandoned" after 4 years, like your good old 2007-2008 MacBooks...
But looking at Apple's latest hardware requirements for some new features, thinking about that I could still install the newest OS X in 2015... only a dream
I also wouldn't be surprised if the poor MBA owners who went with 2 GB ram were left behind next year. So I really hope Mountain Lion will be a stabile and smoothed out OS, as I'm seeing myself and a lot of other people stuck with it... I cannot afford buying a new Mac after every 2-3 years (let alone every year ).