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.)