It seems that Mac developers and power users no longer have the capability to rebuild, replace, and alter the OS X kernel from source code. This means that Linux clearly has the upper hand in terms of openness -- the very same reason why it has become the second most popular OS.

What does proprietary mean? End users generally do not have the ability to run the software for any purpose, study and modify the software, copy the software and provide it to third parties, and make and release improvements to the software.

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 happen to be a proud owner of a G3, G4 or CS Mac, you can hack your own Darwin kernel and use it to boot OS X. However, if you have an Intel-based Mac desktop or notebook, your kernel and device drivers are impenetrable...for now.

Although Apple is largely generous to the open source community by open sourcing internally developed technologies such as QuickTime Streaming Server and Bonjour zero-config networking, the source code required to make a personalized OS X kernel, however, is gone.

What does this mean to Apple users? This step back to a proprietary kernel means that all users must accept a fixed level of performance since they can't configure it by themselves. The default OS X kernels are built for broad compatibility rather than 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.

Apple is afraid of losing hardware sales to software pirates. They do not want to face the possibility of dealing with the selling and distribution of cloned Macs in foreign markets with weak protection for intellectual property. They have all the reasons to be paranoid, but not paranoid enough to slam its doors to the open source community.