Don't hold your breath for the ZFS filesystem to appear in any future Snow Leopard updates. A message yesterday on the project's homepage very clearly acknowledges that development has stopped completely.
A message on MacOS Forge tersely states,
The ZFS project has been discontinued. The mailing list and repository will also be removed shortly.
There have been multiple explanations as to why Apple dropped ZFS support in Snow Leopard, and now there's more speculation to add into the mix. This time the story goes that when Oracle bought Sun, Oracle didn't want continue development on ZFS because they already had their own filesystem (BTRFS) in the oven.
ZFS was also apparently facing patent suits at the time. It all sounds like a complicated legal and political mess, which is probably why Apple just dropped it.
Whatever the true causes behind ZFS' sudden fall from grace may be, it's disappointing that we'll have to wait longer for a true filesystem upgrade in OS X. [AppleInsider]