[mattyb]

In a nutshell, the issue is that the Linux kernel which is licensed under the GNU General Public License is incompatible with ZFS which is licensed under the Sun CDDL. While both the GPL and CDDL are open source licenses their terms are such that it is impossible to simultaneously satisfy both licenses. This means that a single derived work of the Linux kernel and ZFS cannot be legally distributed.

http://wiki.github.com/behlendorf/zfs/faq

FreeBSD has it, but it's been largely abandoned on Mac OS X.