To shut out the OSX on X86 crowd and the filthy pirates who sell hardware running OSX on generic boxes, Apple is no longer releasing the Kernel source for OSX. That means you will not be able to hack and then compile your own Kernel to take advantage of the new improved Intel architectures.

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 customised OS X kernel, however, is gone. Apple says that the state of an OS X-compatible open source x86 Darwin kernel is "in flux."