The bless command in Leopard contains references to ZFS.

kserver:~ pbuffr$ strings /usr/sbin/bless | grep -i zfs
No ZFS container partitions found
ZFS container partition found: %s

Since bless is the command that sets the boot variables in nvram to set your a startup volume, it seems likely that Apple is at least working on boot support for ZFS, if it isn’t already there, but hidden away.  Interesting.