AFAIK, OS X 10.5 have read-only support for the ZFS filesystem. There's some tool released by Apple that gives complete read and write control of the filesystem, but I ignore its utility.
D'oh! I should have clarified that the support would have come with a new release of iTunes that would be released at the same time that OS X 10.5, that is, packed with it. It didn't happen anyway, seems that the only update iTunes receives nowadays is for supporting newer models of the iPod. Not that it troubles me, though