replying to correct myself:
Ciaran O'Riordan ciaran@fsfe.org writes:
This is a non-issue for Wikipedia because each contributor owns their contributions
No, I seem to be wrong. The licence says:
"The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
so the licence gives the site operating the choice to change the licence of content which is owned by the users.
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?