ZFS already has great support on Solaris and FreeBSD. If you're running linux check out http://zfsonlinux.org/ - native kernel support that avoids the GPL incompatibility of Sun's zfs implementation (CDDL license).
They don't avoid it; it's still CDDL, and it's still incompatible. This means that it can't be shipped within the kernel itself, so you have to build a module (which is marked as tainted) for your particular kernel. OpenAFS has this same issue BTW.
