While the whole of the PowerPC flavor of Darwin is open source, parts of the Intel version are not. The kernel and drivers have been made closed source by Apple, presumably to protect against use on generic x86 PCs. Considering Apple’s business model is built upon hardware sales, it’s no wonder that they are pulling out all the stops to maintain profitability.
However, Tom Yager points out some users that will be hurt by this:
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.
Will the Intel version of the kernel stay proprietary? Will it really hurt scientific users? Only time will tell.
Personally, I will never really need to hack the kernel running behind my G4 mac, but now that Apple have made the ability to edit open source somewhat more difficult, it feels like I am being confiscated of the freedom that the mac provides and thrown into something like a windows environment.
I should be worried about this because? Apple has done a better job at making unix user friendly in a matter of years than the entire open source community has done with linux since it's creation. I for one loose nothing by them closing the source.