Can I boot off of ZFS? Not yet but we are working on it. Since booting off of ZFS requires changes to other parts of the system we won't likely be able to release it in a magical tarball. However our goal is to be able to have it available for the next OS X release and sooner if we can.
So will we get it in a 10.5 point release? Given Apple's history with rolling out this kind of feature, it seems unlikely, but you never know.
Personally, ZFS isn't much use to me right now as my sole machine is a one-volume iMac. But if I get a Mac Pro I won't hesitate to fill up the drive bays with disks and format the whole thing as ZFS. All of the technical documentation I've read (not marketing hype or weblog trolling) indicates that ZFS will be much more robust than the flimsy joke that is HFS+, and will provide the best interface (pools) for spreading your data out across multiple physical disks.