Media event set for September 9, launch likely ~10 days later.
ZFS To Become Default File System In Leopard
In fact, this week you'll see that Apple is announcing at their Worldwide Developer Conference that ZFS has become the file system in Mac OS 10.
Rumors of Apple's interest in ZFS began in April 2006, when an OpenSolaris mailing list revealed that Apple had contacted Sun regarding porting ZFS to OS 10. The file system later began making appearances in Leopard builds.
ZFS has a long list of improvements over Apple's current file system, Journaled HFS+. More information on ZFS is available at the ZFS homepage and Wikipedia.
For live coverage of WWDC (text and photos), you know where to turn.