I haven't seen any legal opinion since Canonical's press release, which states that they consulted with counsel who believed that they were in compliance with both licenses.

I -have- seen a lot of argument online, opinions and otherwise that disagree with this, but not anything from a lawyer - mainly "philosophical" or opining on the "intent" of the GPL, rather than "based on clauses x, y, and z, this is impermissible".

Which is not to say, if something of an opinion exists in response to Canonical's release of ZFS, I would love to see it - a legal opinion, not armchair lawyering or opinionizing.