And if we don't, presumably we'd be in the rather odd position of hosting WP material under GFDL only when the original articles on WP are newly licensed under CC-by-sa. Meanwhile, they'd be able to copy CZ material and host it under CC-by-sa but not GFDL... the opposite of what the wikis currently do for new articles (CC for CZ, GFDL for WP).
Although Wikipedia could do that (copy CZ articles), it's unlikely that they would. The proposal is to change WP to GFDL / CC dual licensing. CZ material is only licensed under CC, so it can't be published under the default WP license. It is unlikely that they would create exceptions for a relatively small number of pages.
What about material that started on Wikipedia after November 2008, is copied to CZ under GFDL, and is subsequently copied back to WP after they adopt CC-by-SA and after August 2009? Would this material, modified on Citizendium, be relicensed under CC-by-SA? Or would WP be forced to host material only under GFDL?
The November 2008 deadline is only relevant for works that were not originally published on the relicensing site (in this case, Wikipedia). The material added on CZ do not constitute a separate work, so I think that Wikipedia can import the changed version and relicense it. After August 2009, however, you are no longer allowed to relicense. So the CZ article (GFDL only) can no longer be imported into WP after August 2009 (unless an exception is made for this article; again, unlikely).
By the way, the vote has closed, and the results will be published in a few days.