The last piece of HFS+'s puzzle was FSEvents, an asynchronous file system notification API introduced in Mac OS X 10.5 Leopard. Siracusa detailed FSEvents quite clearly in his Leopard review, and added "as for the file system itself, can you believe we're still using HFS+?" Siracusa stated that HFS+ had to be replaced eventually, and that Apple's work on porting ZFS to Mac OS X could be the key.
Leopard shipped with very basic ZFS capabilities; it only had a basic read-only ZFS driver, despite rumours that ZFS would become the default file system for Leopard. A read/write version was available too at some point, but only to ADC members. Everybody thought that it would eventually find its way into the operating system.
Siracusa was not all that sure that ZFS would be the successor to the venerable HFS+, though. In 2006, he wrote that "although I would be satisfied with ZFS, I think Apple has a unique perspective on computing that might lead to a home-grown file system with some interesting attributes."
It seems like he was right on the money, as news got out today that Apple has closed down the open source Mac OS X ZFS project. The characteristically short and useless notice on the Mac OS X ZFS project page reads: "The ZFS project has been discontinued. The mailing list and repository will also be removed shortly."
This pretty much means that no, ZFS will not be coming to Mac OS X, despite serious efforts by Apple in the past. This is not really something to be surprised about; the first big clue that ZFS would not become part of Mac OS X was when Snow Leopard shipped without any form of ZFS support - not even the read-only driver from Leopard.
Let the speculating commence, I suppose. Previous rumours pointed towards possible licensing issues, but it could also have something to with the NetApp patent lawsuit surrounding ZFS. Coincidentally, Apple posted a job opening for a file system engineer, and John Gruber states that he has heard that Apple is currently working on a home-grown next-generation file system.