This issue has been debated/flamed excessively on Mac OS and Linux fora.
Will Knight has taken a single 1997 quote from Linus's forthcoming autobiography "Just for Fun" about the Mach microkernel which has nothing *NOTHING WHATSOEVER* to do with Mac OS X, plus the fact Linus was offered a job after graduation by Apple which he declined (along with 1e09 other offers by practically every technology company on Earth) and grossly misrepresented it, either through technical ignorance or willful perniciousness. Or both.
As best as I can determine, the facts are these:
1. In the 1980s CMU develops the Mach microkernel, a "pure" microkernel in the sense that everything other than process management runs as a user-process.
2. In 1991 Linus Torvalds began work on a x386 Unix clone based on a monokernel, where most OS processes run in a shared privileged process.
3. In 1997, Linus claims "pure" microkernels are inefficient and unnecessary, with low performance. For an example he cites the Mach microkernel.
4. In 2001 Apple releases a new OS which uses a *modified third-generation version* of the Mach mickokernel with many performance improvements specifically but not exclusively incorporating many *OS processes running as shared privileged processes* in order to avoid the performance problems associated with "pure" microkernels, the same way Linux, Solaris, Windows 2000, and just about every OS not used for research does.
5. Linus decides to publish an autobiography including his earlier comment about "pure" microkernels.
6. Will Knight reads a prerelease of the autobiography, reads the comment about Mach, sort of remembers something about Mach being in Mac OS X, puts 2 and 2 together to get -3.14159, and publishes an irresponsible opinion piece masquerading as hard news about how "Linus sez Mac OS X SUX!".
7. Computer sites which confuse advocacy with fact (including but not limited to ZDNet, the Register, and Slashdot) breathlessly report this unsubstantiated vapor as fact, adding their own free-associations about Linus hating/being threatened by/jealous of Apple. No one bothers to verify any of these remarkable assertions or ask the principals involved.
8. Millions of technology professionals realize the technology press is about as unbiased and accurate as the National Enquirer.