Actually the new kernel on the Mac OS X is a Mach kernel, with some BSD stuff added. It won't scale like Linux, but it wasn't really designed to be a SMP server.
It isn't Linux. It is GNU and *nix compatible. It isn't rpm compatible directly with any of the major distributions, which means that you won't be click-and-installing pre-compiled Linux binaries. I don't have trouble modifying configure scripts or compiling my own software, and you probably don't either, but the average user might. If the user base ever got large enough, that issue might solve itself. As far as I know apps written for the KDE or Gnome window managers won't be compatible, but this might also be a self-solving issue.
I really hope Apple manages to port OS X to the x86 architecture. If they can do that, and get a user base going, they might just have the answer we have all been looking for. I hope they pull it off. It certainly is easier to handle than Linux is, especially for the average user.
I'd love to see OS X in person. I have yet to actually get my hands on a newer machine capable of running the OS.
I guess that the main thing that might keep me from making the OS X switch is the current Linux/KDE apps that I use every day, and am comfortable with. These are pretty uniform across the *nix world. Most of the non-IE or Office stuff type is, in my opinion, vastly superior to any of the Windows stuff comparable with it. Which is why I feel Linux is worth the effort.
Oh, I have IE, Office XP Pro, and DOS all running under Linux, too. This is emulation, of course, and not native, but the speed is fine. And I still have dual-boot capability, if I really need Windows. It is actually kind of neat seeing Windows and Linux and DOS apps all running on the desktop in separate windows, and at the same time. Though I honestly don't have need of the emulation often these days.
And you're right, Linux isn't as plug-and-play as the Mac, but it does amazingly well these days.
I would love to get my hands on Apples big wide-screen LCD monitor. That bad boy is awesome.