It's the other way around. NT and Darwin have a microkernel and Linux has a monolithic. Not too sure if it is a big issue anymore since many of the advantages of a ukernel have been incorporated in Linux, very few things take advantage of the inherent advantages of a ukernel ("Classic" old Windows compatibility), and the slowdown of a ukernel vs. a monolithic gets marginalized as applications get more complex. (Certainly not an "order of magnitude" like the original flamebait claims.)
Honestly, if Darwin were so slow, then why is the #3 fastest supercomputer using it? Certainly there are Linux drivers for the G5 now--heck, 64-bit support is now standard in kernel 2.6.
BTW, Linux is far good for more than just a server. It makes the core of many excellent operating system distributions. It is an excellent and rapidly improving embeddable operating system (in fact, I think it will overtake WindowsCE at the rate its going). It now has even hooks for the beginnings of a serious real-time operating system.