It's critical to remember that we're the 800-pound gorilla in the room of users of the GFDL.
I would bet that statistically speaking, Wikipedia is just about the only user of GFDL. Perhaps also GNU textbooks?
In other words, we should be able to dictate terms (in a nice way) with Stallman to get what we want -- that is, compatibility with CC-SA. Or, as Mav put it earlier, "What we really need is a GNU FDL 2.0 with a clause stating something to the effect of 'Any documents licensed under the GNU FDL 2.0 or any later version with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts can also be licensed under the GNU Free Content License.'"
In the meantime, I recommend a serious project to engage in cross-licensing by a) Changing "You agree to license your contributions under the GFDL." to "You agree to license your contributions under the GFDL and CC-SA." so that all edits henceforth are cross-licensed. b) Encourage the manual cross-licensing project of old contributions: http://meta.wikimedia.org/wiki/Guide_to_the_CC_dual-license#List_of_dual_lic...