It looks like the on againoff again reports of support for the newfangled ZFS file system in Leopard are on again, at least according to the latest word from Mac Rumors, which is citing unspecified "published reports" on the matter. What's more, this apparently isn't the stripped-down, read only incarnation of ZFS we heard about earlier this month, but a full-fledged read/write version, the beta of which has reportedly already been sent to developers. ZFS, for those not up on their file systems, was developed by Sun and promises a range of advantages over lesser file systems, including storage pooling, block-journaling, and other things that a rare few get really excited about.
Update: Reader skwasha notes that apparently the ZFS Leopard files have been pulled, but produced a document regarding the ZFS dev preview, which states: "ZFS is a new filesystem from Sun Microsystems which has been ported by Apple to Mac OS X. The initial (10.5.0) release of Leopard will restrict ZFS to read-only, so no ZFS pools or filesystems can be modified or created. This Developer Preview will enable full read/write capability, which includes the creation/destruction of ZFS pools and filesystems." So there ya go.