ZFS is on Leopard... Sorta.
"ZFS is not the default file system for Leopard. We are exploring it as a file system option for high-end storage systems with really large storage. As a result, we have included ZFS -- a read-only copy of ZFS -- in Leopard."
This means that Leopard will be able to read ZFS volumes... assuming there was a way to write to one on Mac OS X, which there isn't... yet.
"Read-only means that at a later date, if there are ZFS volumes, those systems would be able to read ZFS volumes," Croll added. "You cannot write data into the system. It will allow you to read ZFS volumes later."