Well it was a while back so my memory could be failing me, but I read that NeXT OpenStep contained a Mach kernel based off an early version which was not in practice a micro-kernel (iirc Mach development was long and problematic and relied on a monolithic design for much of it's functionality during most of it's development cycle) together with parts from FreeBSD and NetBSD from which then emerged OSX.
The story I read was an article series which was a rebuttal of people assuming that OSX was slow due to having a micro-kernel, which the author (while also providing technical insight which seemed to back him up) claimed it did not. I'll see if I can find it again, it was quite some time ago I read it.
edit: I think I found the article series about Next/Mach:
http://www.roughlydrafted.com/0506.mk1.html
Here's hoping I remembered it correctly, else I feel confident Awesomness will be there to 'correct' me
