Intel (x86) OS X Kernel Becoming Closed-Source?
According to an opinion piece by Tom Yager at Infoworld, Apple has closed the source of the Intel version of the Mac OS X kernel (called XNU) due to piracy concerns. While the rest of the underlying operating system of OS X (called Darwin) is still open-source, closing the kernel means less capability for pirates to hack OS X to run on non-Apple hardware, but also limits developers and power-users' ability to tweak their systems.
Apple's Open-Darwin mailing list has an ongoing discussion in which some users point out that Apple has actually not released the XNU source for Intel since 10.4.3 or perhaps earlier. Therefore, it appears as though the facts of the article are not new, but rather are just now gaining press attention.
Apple has not officially commented on the future of XNU's source availability, and it appears most conclusions as to why the source has not been released is based on speculation rather than inside knowledge.