We at Canonical have conducted a legal review, including discussion with the industry's leading software freedom legal counsel, of the licenses that apply to the Linux kernel and to ZFS.
And in doing so, we have concluded that we are acting within the rights granted and in compliance with their terms of both of those licenses. Others have independently achieved the same conclusion. Differing opinions exist, but please bear in mind that these are opinions.
While the CDDL and GPLv2 are both "copyleft" licenses, they have different scope. The CDDL applies to all files under the CDDL, while the GPLv2 applies to derivative works.
The CDDL cannot apply to the Linux kernel because zfs.ko is a self-contained file system module -- the kernel itself is quite obviously not a derivative work of this new file system.
And zfs.ko, as a self-contained file system module, is clearly not a derivative work of the Linux kernel but rather quite obviously a derivative work of OpenZFS and OpenSolaris. Equivalent exceptions have existed for many years, for various other stand alone, self-contained, non-GPL kernel modules.
Our conclusion is good for Ubuntu users, good for Linux, and good for all of free and open source software.
EDIT: This post was updated to link to the supportive position paper from Eben Moglen of the SFLC, an amicus brief from James Bottomley, as well as the contrarian position from Bradley Kuhn and the SFC.