In 10.5, Apple released a read-only kernel extension/filesystem to allow ZFS pools to be mounted. A later open-source project was hosted at http://zfs.macosforge.org and was announced to be part of 10.6, but shortly before the public release ZFS was yanked, and recently, the Apple-hosted MacOSForge site has closed down the ZFS project.
So, what's the status of ZFS on Mac OS X? Is it worth using? Why would anyone be wanting to use ZFS anyway?
There have been many speculated reasons on the demise of ZFS on Mac OS X. About the only thing that you can say is that (a) it has been deprecated, almost certainly for good, and (b) that no-one knows the real reason why, or at least, isn't saying.
Update Jeff Bonwick, creator of ZFS, suggests that it was a lack of an agreement on (commercial) licensing that was the blocker.
Answers from lawsuits and patent litigation to excessive licensing fees and the impact of the Snoracle deal have been suggested; another is system stability. However, the why is somewhat unimportant, since the end result is the same - Apple no longer support ZFS on Mac OS X.
Does this mean this is the end of ZFS on Mac OS X? Not necessarily. Before the project was yanked, enterprising individuals made copies of the SVN repository (on GitHub Peaceful, Dustin, AlBlue) and of the last publicly available installers and installers were cached on the google group zfs-macos).
A new Google Code project has been set up to take over the maintenance and on-going development of the ZFS extension, and a mailing list exists with 90 members, mostly from the old Apple-hosted list.
The previous ZFS builds works on Leopard and a new build for the 64-bit Snow Leopard has been made available. Work will continue and hopefully port forward some of the later enhancements to ZFS that have been made on the OpenSolaris group since the Mac fork occurred at build 72.
Whilst Apple is looking to recruit new filesystem developers, and is likely looking to extend the increasingly crufty HFS or take lessons learned from ZFS and apply those to the next generation filesystem, probably making its appearance in 10.7 or afterwards.
In the meantime, the ZFS project continues to live on in a new location.
As an update to this, ZFS binaries have been made available from the old Apple build and ported to Snow Leopard, and many people are using these already. The code has been synchronised with the upstream OpenSolaris codebase, and more recently, Apple has released the CDDL parts of their previously available ZFS port to developers. So it looks like for the Snow Leopard timeframe at least, ZFS will live on on Mac OS X.