In the last few days Apple has released "ZFS on Mac OS X Preview 1.1" as well as support documentation to developers. Within the documentation Apple acknowledged that it has single handily ported ZFS to Mac OS X Leopard.
Apple also confirmed that although this developer preview enables writing capabilities the final release of Leopard will be restricted to a read-only implementation.
Developers receiving the latest ZFS preview, however, are granted access to full read and write capabilities under Leopard, including the ability to create and destruct ZFS pools and filesystems.
World of Apple uncovered Apple's implementation of ZFS in December 2006.