Mac developers and power users no longer have the freedom to alter, rebuild, and replace the OS X kernel from source code. Stripped of openness, it no longer possesses the quality that elevated Linux to its status as the second most popular commercial OS.
The Darwin open source Mach/Unix core shared by OS X Tiger client and OS X Tiger Server remains completely open for PowerPC Macs. If you have a G3, G4, or G5 Mac, you can hack your own Darwin kernel and use it to boot OS X. But if you have an Intel-based Mac desktop or notebook, your kernel and device drivers are inviolable. Apple still publishes the source code for OS X's commands and utilities and laudably goes several extra miles by open sourcing internally developed technologies such as QuickTime Streaming Server and Bonjour zero-config networking. The source code required to build a customized OS X kernel, however, is gone. Apple says that the state of an OS X-compatible open source x86 Darwin kernel is "in flux."
Apple has only shipped client systems, the users of which care least about openness. Soon, though, Apple will break out Intel variants of the kinds of machines that InfoWorld readers buy and on which I depend; namely, servers and workstations. I hope that Apple's flux settles into a strategy that favors demanding users and developers.
Apple's retreat to a proprietary kernel means that all users must accept a fixed level of performance. The default OS X kernels are built for broad compatibility rather than breakneck speed and throughput. That doesn't matter at present, because all Intel Macs are built on the same Core Duo/Core Solo 32-bit architecture. But Apple's workstation and server will be built using next-generation 64-bit x86 CPUs. The chipset, the bus, the memory, almost everything about the high-end machines will be much advanced over iMac and MacBook Pro. Intel's road map plots a rapid course to ever higher performance. Macs will inherit the benefits of Core Microarchitecture's evolution, but OS X is limited in the degree to which it can exploit specific new features without creating branch after branch of OS code to handle each tweak to the architecture.
Users in demanding fields such as biosciences or meteorology do hack OS kernels to slim them down, alter the balance between throughput and computing, and to open them to the resources of a massive grid. The availability of Intel's top-shelf compilers, debuggers, libraries, and profilers create unprecedented opportunities to optimize OS X for specific applications.