That is great explanation from Linus. Thanks for linking it. He uses AFS (Andrew File System) as an example in that post which I think would be very analogous to ZFS. Linus says

  > Historically, there's been things like the original Andrew filesystem
  > module: a standard filesystem that really wasn't written for Linux in
  > the first place, and just implements a UNIX filesystem. Is that
  > derived just because it got ported to Linux that had a reasonably
  > similar VFS interface to what other UNIXes did? Personally, I didn't
  > feel that I could make that judgment call. Maybe it was, maybe it
  > wasn't, but it clearly is a gray area.

  > Personally, I think that case wasn't a derived work, and I was willing
  > to tell the AFS guys so.

So, the ZFS case is not cut and dried either way. It would probably take a court case to clarify the matter as it is in a "gray area". To me, there seems like there is a good chance than ZFS and the Linux Kernel can be distributed as a combined work (But I Am Not A Lawyer!).