ZFS Read/Write Developer Beta for Leopard
It appears that Sun's Jonathan Schwartz may have known what he was talking about when he claimed that ZFS support was coming to Mac OS X at WWDC.
WWDC came and went, however, with no ZFS announcement from Apple. Instead, Apple's Brian Croll flat out said ZFS is not happening. He later clarified that ZFS for Leopard was present but "read-only" -- meaning that Leopard was unable to write data to ZFS volumes.
Now, according to published reports on the internet, Apple has actually seeded developers with a ZFS Read/Write Beta for Leopard. The new ZFS Read/Write beta enables full read/write capabilities to Apple's Mac OS X (Leopard).