Have you read the debate between Andy Tanenbaum and Linus Torvalds from about 1992? It's a discussion of the tradeoffs between microkernels (Tanenbaum) and monolithic kernels (Torvalds). You can find it here:
The claim that microkernels improve portability makes no sense to me, since Linux itself is very portable, as are the BSDs (especially NetBSD and OpenBSD).
http://oreilly.com/catalog/opensources/book/appa.html
The chapter by Torvalds in the same book is also worth reading:
http://oreilly.com/openbook/opensources/book/linus.html
The claim that microkernels improve portability makes no sense to me, since Linux itself is very portable, as are the BSDs (especially NetBSD and OpenBSD).