Apple appears to have pulled the publicly accessible Mac OS 10.4.8 Source Code (Darwin, the open-source foundation of OS X, and XNU, Darwin's open-source kernel), leaving only developers with ADC log-ins with access to the code (public link, ADC link)
Earlier this week, the OSx86 project released a version of the 10.4.8 kernel that was hailed to be 100% legal according to the APSL and run on any x86 machine.
Prior to this release, Apple's code would only run on Apple's hardware due to various dependencies (such as EFI).
While the released code from OSx86 can run under single-user mode on any x86 machine, it will not boot up into Apple's familiar OS X Aqua interface without Apple's TPM keys which are illegal to distribute (but are reportedly floating around the internet).
The circumstances are reminiscent of when Apple previously stopped releasing the Intel version of the Mac OS X kernel, which some had concluded was due to piracy concerns. Apple finally released the source at WWDC along with an apology for the delay, but did not give an explanation.
Update (moved to page 2): Certain aspects of the story, i.e. the points regarding the history and what exactly happened with the Darwin/XNU source, have been called into doubt.
Kernel coder Semthex made the above claims on his blog, but MacRumors forum member shawnce has called the claims into doubt in this story's thread.
The end-fact that Apple's source does not appear to be publicly accessible does not appear to be in dispute, but rather questions have arisen as to whether it was ever publicly accessible (publicly accessible as defined by not requiring registration with Apple. The source is still free).