I wouldn't say OS X is a microkernel. Much of its codebase was built off of Mach, but much was also built off of BSD. For example, its network stack logic resides in the kernel, which is certainly not microkernel-esque. See http://en.wikipedia.org/wiki/Darwin_(operating_system)#Kerne...

"Mac OS X is sort of microkernelish. Inside, it consists of Berkeley UNIX riding on top of a modified version of the Mach microkernel. Since all of it runs in kernel mode (to get that little extra bit of performance) it is not a true microkernel, but Carnegie Mellon University had Berkeley UNIX running on Mach in user space years ago, so it probably could be done again, albeit with a small amount of performance loss, as with L4Linux. Work is underway to port the Apple BSD code (Darwin) to L4 to make it a true microkernel system." - Andy Tanenbaum

reference - http://www.cs.vu.nl/~ast/reliable-os/

I'm not talking about just micro- vs macro- kernel. I'm talking about general OS-level research. Just look at what MSR is doing with Singularity: http://research.microsoft.com/en-us/projects/singularity/

OS X is not a microkernel and hasn't been one since 10.0. It is a hybrid kernel called the XNU kernel. It's Mach microkernel based with some low layer BSD smothered on top.

It would be interesting to see how the original Mach microkernel runs on modern hardware. At the time, the context switching of the Mach kernel was making performance poor. But this was on 300/400 MHz machines.