apple laptops ≠ linux machines. in fact, most of the new macbooks are quite hard to get running on something modern like ubuntu 10.4 LTS, and just because OSX is a derivative of BSD doesn't make it friendly with current linux versions. especially those macbooks with "automatic graphics switching" and the like. sure you can boot, but all the [proprietary] advanced [closed] features that apple adds don't really function at all.
I think the reason that people suggest Mac over Linux for laptops is that 1. Apple makes really good laptops. They're solid, they're lightweight, they're not creaky and they don't feel cheap; 2. You can do anything on OS X that you can on Linux (e.g. running bash, RoR, Python scripting, most open-source software).
This is a big part of the reason geeks are moving in large volumes to the Mac: you can have your laptop, you can compile and run Apache or PHP, install gems, test Rails, and it's all just as effortless as in Linux (or, depending on the Linux distro, even more so).
What you don't have to do is re-configure X or your nvidia drivers when a 'yum update' does a kernel upgrade, and you suddenly find yourself without X for ten minutes while you find and run the nvidia kernel module builder/installer (a problem everyone in my office had every few weeks at one company).
A lot of stuff just works effortlessly, and while this is getting better in Linux, there's still a lot of 'gotchas' that crop up out of nowhere because some company or some developer just doesn't care.
It's not a question of 'You want Linux, buy a Mac and run Linux on it', it's a question of asking why the user wants Linux. If it's so that all your software, top-to-bottom (excluding the BIOS) is open-source, then get a Lenovo, but if what you want is a better Rails dev environment than Windows, a Mac will do that for you and be less effort in the long run.
This is the reason I'm on a Mac, and it's the reason a lot of other open-source or web app developers run Macs as well, and I think it's the secret sauce that's made a big difference in Apple's sales numbers - geeks buy, then geeks recommend.
linux is great on a 2 year old macbook, however.