ZFS was in fact promised for 10.5 and was even featured prominently on its specs page with a complimentary graphic and all. It was quietly removed some point before 10.5 was released.

I can see an opening for a lawsuit against Apple here, since the advertising for 10.6 wasn't disclaimed as a forward-looking statement that may or may not come to fruition.

Being pissed at Apple aside, there's still a pending lawsuit against Sun and ZFS by NetApp: http://www.sun.com/lawsuit/zfs/

I'm sure Apple wasn't too pleased to be working on a technology facing possible litigation - which has been going on since September of 2007. This also appears to be Sun's fault as they provoked NetApp into filing this defensive lawsuit when they went after them seeking royalties for some of their own patents.

Meanwhile Apple is apparently working on its own next generation filesystem and currently even have one (or more?) outstanding job requisitions for a related position.