Recently, an interesting discussion came up about the difficulties getting ZFS on GNU/Linux. It's over in the zfs dicussion forum, but I don't really want to link to it as it got a bit heated at times. But, between the heat, what came out was that it'll be pretty much impossible to get ZFS ported to GNU/Linux, given the current understanding of the GPL.
I think ZFS is great, and I'm not just saying that because I'm on the team - I only joined a few months ago and I thought it was great well before then. I love playing around with it in Solaris. I love the fact that it's in FreeBSD, and there are also rumours of ZFS in OS X Leopard. I'd love to see ZFS on Linux. Heck, I'd love to see it in Windows. Imagine doing a full install of XP SP2, snapshotting C:, cloning it, and then running right off the clone. When the system eventually becomes unstable, make a new clone and blam you're back in action.
What came out of the discussion (and please correct me if I misunderstood) was that to get ZFS into the Linux kernel, it would require it to be GPL'd. The kernel can't call ZFS functions without ZFS being under the GPL. This is unfortunate - I don't see ZFS being GPL'd any time soon. One person in the discussion brought up the possibility of creating a layer of indirection between the kernel and ZFS, but even that seems doomed: the abstraction layer itself would have to be GPL'd so the kernel could integrate with it, and then for the layer to call ZFS, ZFS would still need to be GPL'd.
The GPL has been a great tool for the open source community. It's too bad that it leaves no room for compromise.