Forgive me Ciaran, but I think you're pretty much begging the question:
Ciaran O'Riordan wrote:
Knowing FSF, I assume they got sufficient reassurances from Wikimedia Foundation that they won't abuse this power. So to assess the risks, the question to ask is what other sites (GFDL'd wikis) are eligible for this change of licence?
The reassurances from Wikimedia, whatever they were, is by-the-by really. The point is this; if you contribute to a project under the GFDL, are we really saying that CC-BY-SA is the spiritual ancestor of that license?
I personally find that a difficult proposition.
This appears to be a pragmatic approach to solving a license incompatibility. The truly pragmatic approach, though, would be to get rid of one or other of the licenses, so that one was truly the ancestor of the other, and reduce fragmentation. That's not happening, though: the incompatibility will remain, but a small window will open up when people can choose a license other than that set by the author. In that respect, it's a temporary band-aid over the problem.
If it was a permanent solution, I might have fewer problems with it. But what message does this send out to authors? "To solve proliferation problems, we'll relicense your works when we see fit?". This GFDL->CC-BY-SA process could be all sweetness and light, and cause nothing but good. That's not really the point I raised.
Putting it concisely: on what basis is CC-BY-SA a spiritual successor to the GFDL, other than the technical fact that it has been grand-childed in?
Cheers,
Alex.