There was a reason ZFS wasn't named-checked as one of Leopard's 10 (not so) new features at WWDC 07—despite declarations by Sun's CEO, Jonathan Schwartz, yesterday Apple denied ZFS's presence in Leopard. Put another way by Brian Croll, senior director of product marketing for OS X: "ZFS is not happening." Ouch.
But if you read into it and consider InfoWeek's note, "upon further questioning, Croll would only confirm that Apple had never said ZFS would be a part of Leopard," in conjunction with the fact that it was Sun's CEO making the slip, not some random peon, it seems fairly likely that something Sunny was cooking in the Apple kitchen. It's not like they're down and out in any case—Apple's probably already kicking around stuff for 10.6
Apple Says No Sun File System For Leopard [InformationWeek]
"ZFS is not the default file system for Leopard. We are exploring it as a file system option for high-end storage systems with really large storage. As a result, we have included ZFS — a read-only copy of ZFS — in Leopard."