The FAQ seems to say we explicitly will not be doing any of this license tracking: "It will be the obligation of re-users to validate whether an article includes CC-BY-SA-only changes -- dual licensing should not be a burden on editors. This is also not intended to be bidirectional, i.e., merging in GFDL-only text will not be possible."
In practice, that means that everything will probably need to be treated as SA-only, unless a reuser wants to validate for themselves that they can legitimately use it under the GFDL. I personally see that as a feature, but your mileage may vary. =] In particular, having significant amounts of content dual-licensed makes it exceedingly easy for reusers to poison-pill their derived works so we can't re-merge their changes back into Wikipedia, by choosing only the GFDL for their derived works. Of course, that in itself would be no reason to reject the agreement, because the current situation is just as bad: the single-licensed GFDL is inherently designed to permit poison pills via the addition of invariant sections.
The way I see it, that's equivalent to not being dual licensed at all, since there is no way for a re-user to know what licenses things are under unless we tell them. (However, I think we need to tell them in order to comply with the licenses ourselves, so the whole thing is moot.)