My personal anecdote is more that I have a handful of better computers in the house, but the laptop I spend the most time working and programming with is my crappy 8-year old gateway running Ubuntu. I'm not entirely sure why I'm drawn to it over the others, even if it's the slowest and oldest machine in the house (excluding my netbook). But I type faster on this keyboard than any other keyboard I've ever used.
There's just something about it that I prefer, and I'm perfectly willing to deal with its warts, as it's used almost exclusively for programming (vim and ssh don't exactly use a lot of resources). Chrome does have some memory issues on it (and I'm considering switching back to Firefox for that laptop), and sometimes compilation times get annoying, but overall, this system has been tweaked and configured to be my ultimate programming machine.
Plus, testing server software on slower machines can be helpful to find bottlenecks that might not present themselves as obviously on blazing fast machines.
All that said, I'd say we've gotten to the point where a casual user will do just fine with 4-year-old technology. Most Core2 machines are fast enough for just about anything. And Casual PC users typically aren't PC Gamers, which is its own segment, and even then, my Core2 PC for gaming is "fast enough" to play the games I play.