A bit of a follow-up to this question: If the FSF modifies the GFDL to be compatible with CC-BY-SA, wouldn't that negate the need to migrate from one to the other? If the two licenses were fully-compatible to the point that migration was possible, wouldn't we already be getting everything we want out of the GFDL anyway? If the GFDL is modified to suit our needs as well as the CC-BY-SA does, why is there a need to migrate?
So that things already released under CC-BY-SA can be used in GFDL works and vice versa. What form the "migration" will take, I have no idea, though. We need to wait and see what the end result actually says.