There would seem to be good points on both sides, so until it gets
tested in court, I don't think we'll really have an answer. The issue
is also discussed, though without a clear conclusion, in Wikipedia's
article on the GPL [1].
Personally, I don't really like the FSF position (at least not to the
extreme they push it). Everyone agrees that copying code is generally
derivative but counting nearly all form of code interaction as also
derivative feels unnatural. In this circumstance -- and irrespective
of what the law actually says -- it would seem more natural if
software were treated more like a physical object subject to first
sale doctrine and similar principles. For example, it seems more
natural (to me at least) to say that an end-user should have arbitrary
authority to modify and combine legally obtained software, but not be
allowed to distribute copies of the modified software. To give an
analogy, car companies do not have the authority to restrict
after-market modifications to the chassis of a car, and there is a
booming industry of people providing after-market mods. So why should
software copyright holders get to control the skins used in
conjunction with their software?
That said, a lot of the copyleft movement has operated on the
assumption that copyright holders have nearly arbitrary freedom to
impose restrictions on the users of their works. And maybe that is
true. For the most part it hasn't been directly tested. I don't know
whether the FSF is right about the reach of the GPL with respect to
APIs and linked code, but personally, it feels like just another piece
of evidence that copyright law needs to be updated to deal with the
realities of the digital age.
If there is a decision to be made (and I'm not sure there is), I would
personally argue that we should take steps to make sure that authors
are informed about the FSF position on the GPL, but not try to enforce
it ourselves (except possibly for the extensions and skins actually
deployed by Wikimedia). I don't see much benefit for Wikimedia in
taking a hard line with third-party software contributors over the
licensing of their extensions.
[1]
http://en.wikipedia.org/wiki/GPL
-Robert Rohde