Apple has released a new version of Darwin, the Open Source kernel that is the heart of Mac OS X. Darwin is, according to Apple:
What is Darwin? Darwin is the core of Mac OS X. The Darwin kernel is based on FreeBSD and Mach 3.0 technologies and provides protected memory and pre-emptive multitasking. Darwin runs on PowerPC-based Macintosh computers and a version is also available for x86-compatible computers.
As for the new 1.4.1 release, Appleis Darwin Web page says simply that "This release corresponds to Mac OS X v10.1. A Darwin/x86 ISO image is also available." You can find links to all of the Darwin resources at Appleis Darwin site.