Things are a little different when you buy top-end products. I had a ThinkPad T43p which I was quite happy with from 2005-2009, and a T21p for a similar length of time. MBP2010-17" is also probably going to last until 2014 for most of the reasons mentioned in this article (and I'd considered the M6400 at the time).

Anything but top spec stuff means I'd want to switch after a year or two, which if you include the time to do so, is probably more expensive.