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You assert that the inclusion of Y does not make Z a derivative work according to GPL, since you could remove Y and replace it with something else (say, W). That by itself is not true, since the removal of Y altogether would cause functional breakage of Z. If it were true, then you could make a Linux kernel with a non-free binary blob driver on the grounds that because you could pull the driver out and replace it with another that used the same interface, its inclusion in the kernel does not create a single work. That is untrue, and more importantly, the Fd.o icon theme spec does not work by harcoding file names of icons.
However, even if your original assertion (that Z is not a combined work under GPL) was true, the conclusion would still be wrong, because Z is also subject to the terms of CC-BY-SA, and the combined work is also definitely a combined work under CC-BY-SA.
Granted that there is no one, bright line agreed upon by everyone as to what methods of referencing an icon constitutes creation of a derived work, but hardcoding is definitely at the extreme end of the spectrum and thus the least safe. Still, the example also illustrates the other problem: that when combining works of two different licenses, both must be satisfied, and even when things are clear under the terms and definitions of one, they may not be under the terms and definitions of the other. Thanks for providing the example.
[Modified by: Nathan Willis on September 27, 2007 11:24 AM]