"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
I realize that. On the same note, GNU HURD isn't technically a 'micro-kernel' because GNU Mach is the micro-kernel, but my point was that 'micro-kernel' instead of 'monolithic kernal' isn't some unexplored territory that only exists as theory in CompSci textbooks.
reference - http://www.cs.vu.nl/~ast/reliable-os/