It is worth noting that not even Sun fully has ZFS working.
It is one of those things that it can be used for storage, however not booting.
Sure this is mostly because their kernel and boot loader haven't been fully rewritten to allow for it, much like most other "bleeding-edge" filesystems.
While Apple does have their very own BIOS replacement which allows them to do quite a bit more than Sun would be capable of, however they are still limited by the specifications that Sun has published.
I personally have been holding out a lot of hopes for this, that Apple would get ZFS to work directly through modifying their EFI and OpenFirmware implementation, in turn bypassing all the craziness that Sun is needing to jump through. It seems that this is simply not going to happen for Leopard though.
It doesn't mean that they couldn't allow for read/write volumes in a Leopard update, however this is one of those things that screams "MAJOR NEW FEATURE", so would wait for 10.6.