Several sites are reporting on ZFS support that is now built into the latest build of Mac OS X (Leopard) which was seeded last week.
The ZFS team at Sun was contacted by Apple in April about porting ZFS to Mac OS X. Arstechnica previously discussed advantages of ZFS as a file system. These include
- Efficient storage and handling of very small files.
- Logical volume management through a pooled storage model. (Example/Description)
- Improved data integrity using checksums on all data.
- Snapshots
Mac4Ever first posted a screenshot of the ZFS formatting option now in Leopard:

More information on ZFS is available at the ZFS homepage and Wikipedia.
Update: While ZFS support appears to be preliminarily in the newest Leopard builds, if you try to format a disk image using ZFS, the formatting will not complete. This is a known bug.