Much has happened since I asked this question in October 2010.
As of September 2013, a new collaboration known as OpenZFS will serve as a central site for several ZFS projects. The new site is http://open-zfs.org/ (with a dash)
Today at LinuxCon North America, Brian Behlendorf and Matthew Ahrens are announcing that members from the illumos, zfsonlinux.org, FreeBSD and MacOSX ZFS communities have created a project called "OpenZFS" to combine their efforts.
Here is the announcement to the FreeBSD community from FreeBSD Foundation President Justin Gibbs, and the announcement to the illumos community from Matthew Ahrens at Delphix.
Brian Behlendorf (creator of zfsonlinux.org) announced this on firstname.lastname@example.org today:
From: "Behlendorf, Brian D." <behlendorf--->
To: "email@example.com" <firstname.lastname@example.org>
Subject: [zfs-announce] OpenZFS
Date: Tue, 17 Sep 2013 16:46:40 +0000
Today we announce OpenZFS: the truly open source successor to the ZFS
ZFS is the world's most advanced filesystem, in active development for
over a decade. Recent development has continued in the open, and
OpenZFS is the new formal name for this open community of developers,
users, and companies improving, using, and building on ZFS. Founded
by members of the Linux, FreeBSD, Mac OS X, and illumos communities,
including Matt Ahrens, one of the two original authors of ZFS, the
OpenZFS community brings together over a hundred software developers
from these platforms.
You can read more about OpenZFS at our website: http://open-zfs.org
(don't forget the dash!)
Old news from December 2012:
Oracle is still pretty closed about it's ZFS development.
As ZFS outside of Oracle is concerned, the primary upstream seems to be illumos. The major players in the non-Oracle ZFS scene all seem to be collaborating on the illumos kernel, which provides ZFS.