Also of note:
This ZFS on Linux port was produced at the Lawrence Livermore National Laboratory (LLNL) under Contract No. DE-AC52-07NA27344 (Contract 44) between the U.S. Department of Energy (DOE) and Lawrence Livermore National Security, LLC (LLNS) for the operation of LLNL. It has been approved for release under LLNL-CODE-403049.
Thus, typical ZFS users cannot be sued for infringing on the patents that Oracle holds on ZFS technology. However, this does not preclude third-parties (e.g., NetApp) from pursuing patent claims that they believe are infringed by ZFS -- but this is just as true with any other technology (think MS suing Garmin over FAT in Linux).
Liability for infringement on Oracle's ZFS patents is possible if a user does not comply with the terms of the CDDL. For example, modifying the ZFS source code and distributing it using a license other than the CDDL would be a breach of the license terms (§3.1), and thus forgo any patent license. The Apache 2.0 license has similar terms regarding patent rights.
(think MS suing Garmin over FAT in Linux)
Most features are done and you can install with btrfs out of the box today in the RHEL 6 or Centos 6 betas.
Nope, they just have to be compatible with the GPL. You could, for example, release a kernel module under the BSD or ISC license and be in the clear. The problem is that the CDDL is intentionally incompatible with the GPL.
GPL people have a tendency to forget that GPL is a copyright license, and so if someone is doing something that doesn't require copyright permission GPL is irrelevant.
While it obviously can't be distributed with the mainline kernel due to licencing issues, it can be distributed standalone to work with Linux, or any other hypothetical system that provided the same API.
As such ZFS on Mac OS X is a dead project.