According to a MacRumors developer source, ZFS has been added to the list of file systems in the vnode.h header file in Mac OS 10.5 Leopard, which could indicate preliminary support for the file system. Apple had contacted Sun earlier this year about the possibility of porting ZFS to Mac OS X.
Due to ZFS's high redundancy features, some in the technical community had wondered whether Leopard's Time Machine was simply a front end to a ZFS implementation in Leopard. At this point, it appears as though Apple is still using HFS+ as its default file system. ArsTechnica presents a decent up-to-date summary of ZFS and how it may impact Mac users.