http://www.apple.com/macosx/jaguar/unix.html and afaik, 10.1.5 was still FreeBSD 4.2 modified...now 10.2.x is FreeBSD 4.4 and 10.3 when it's released is rumored to use FreeBSD 5 which is currently staged at beta.
According to Apple, pre-Jaguar Mac OS X was based on FreeBSD 3.2, and the 10.2 release jumped it up to FreeBSD 4.4. Note that some elements, including gcc, are even more recent now, which is cool!