Joel Spolsky's article on Biculturism suggests two things to me:
Joel completely ignores the embedded and mobile markets: Symbian, PalmOS, QNX, VXWorks, Epoc and Windows CE are all used on mobile and embedded devices and give many programmers exposure to operating systems other than the big two. Some of those are based on Posix or Win32, some are quite different, and some are both. QNX, for example is a Posix API above a tiny, network transparent, message passing microkernel.
Rob Pike's comments are very true though. There is so much good OS research that has not made an impact, or even a ripple. Many ideas from recent OS research would be ideal for mobile, networked computers: transparent persistence, capability based security, JIT optimisation of the OS on a per-program basis, etc.
I cut my professional teeth on HP minicomputers running the MPE/3000 OS. However, I'm quite happy programming on Unix or Windows. I'd rather use a pleasant high level language and drop into C when necessary, or even program in Java, than write another line of COBOL!