<quote who="SJ Klein" date="Sun, Dec 02, 2007 at 02:22:41PM -0500">
My understanding, having talked to several lawyers about this issue, is that this "difference" hangs on interpretation of case law about what constitutes a derivative work. The answer to the question is incredibly ambiguous and jurisdiction specific.
It is, in fact, a matter of extra-license legal definitions and not of license or what CC/FSF/SFLC thinks. The FSF and CC each have positions on this that, in various jurisdictions, are each contradicted by existing case-law. Neither license says anything in the text of the license and neither plans to.
If the author of a work under BY-SA subscribes to the more expansive definition of derivative work and litigates in a jurisdiction that is friendly to it, they'll probably have luck. If they use the GFDL and are in a jurisdiction that has precedent saying otherwise, they won't.
There is, as I have come to understand, no difference between the licenses in this regard that would prevent compatibility.
That is most interesting.
However, to the point above, one could still make a license for photographers which expressly states that one can reuse a photograph only if the surrounding text and other media (defined separately from 'derivative work') are available under a similar license.
The license [call it SA-X] under which the photograph itself is licensed would change. the copyleft clause would be passed on through derivatives and bundling (or however you name combining text with an image), and could specify a set of licenses that could be used for surrounding media.
This need not pass on the 'bundling' clause to the surrounding works. could be compatible with both by-sa and gfdl, whose notion of further compatibility is related to derivatives.
Until I've thought about this more and talked to some lawyers, I won't speculate on how this might work or what legal barriers to making this work might exist. That said, I'm excited by the possibility.
I was not trying to imply that such a strong copyleft license was impossible, only that in the current situation the GFDL and CC BY-SA are effectively identical in their (in)effectiveness in this regard.
Regards, Mako