The urban legend is that Sun invented the CDDL explicitly to prevent GPL-licensed competitors, Linux in particular, from benefiting from their work. I've not studied the CDDL at all, but I never hear anyone claim there's any other benefit to it.
I'm not sure what the rightsholder situation is with regard to ZFS (that is, I don't know if they had a copyright assignment for contributors), but it'd be really interesting if someone could convince Oracle, who I assume is the owner, to re-license it under the GPL. Oracle seems to have at least equal commercial interest in Linux as Solaris at this point. I'm not sure what the benefit is in keeping the license incompatible anymore.
Oracle used to have a copyright assignment policy that covered ZFS contributions, but it went away when they officially killed OpenSolaris. Any contributions since then have been under CDDL and the standard copyright mechanism where authors retain ownership is operating as normal.
It's true that Oracle can issue an update to the CDDL that would automatically apply to (past) versions of ZFS, since the CDDL delegates to them (nee Sun) as the license steward and ZFS was being distributed with "or later" terms. Last I checked, though, the current ZFS project maintainers decided to patch that by opting out of the "or later" terms for future versions. At least this was true when I spoke to ryao about it last year.
This was around the time that there was a lot of attention on ZFS on Linux. Eben Moglen did a compelling writeup at that time arguing that ZFS is most likely freely mixable with GPL nowadays, because users have been able to accept it on GPL terms ever since Oracle has shipped Linux versions that include ZFS and doing so without making it available under GPL would be a violation of the kernel's rightsholders' copyright.
CDDL is still a total quagmire. I'll repeat: there's no good reason to adopt CDDL today, and there's lots of reasons to avoid it.
The real "legend" is more that they were trying to open source something that was riddled with proprietary 3rd party stuff. So they had to get around it.
I'm not sure what the rightsholder situation is with regard to ZFS (that is, I don't know if they had a copyright assignment for contributors), but it'd be really interesting if someone could convince Oracle, who I assume is the owner, to re-license it under the GPL. Oracle seems to have at least equal commercial interest in Linux as Solaris at this point. I'm not sure what the benefit is in keeping the license incompatible anymore.