> Harald Welte of the netfilter/iptables core team sought to enjoin Sitecom from distributing its WL-122 router, which used netfilter's GPL'd code, without also providing the source code and a copy of the GPL, as that license requires
and
> The Munich Court granted Welte a preliminary injunction [2] and then upheld that injunction [3][4]
I believe there are other instances of GPL being upheld in courts around the world and this one should be only one of many examples.
GPL is not even necessary in these cases, in the absence of the (GP) license it reverts to the default copyright rules, with all rights of copy and distribution being at the hands of the copyright holders (save for Fair use and other exceptions).
A company that would try to argue that GPL is not a valid license in court would actually be admitting in court that they are distributing the software without a license from the copyright holders all along.
[1] Munich Court Again Enforces GPL: http://news.slashdot.org/story/05/04/14/2024258/munich-court...
[2] Preliminary injunction: http://yro.slashdot.org/article.pl?sid=04/04/15/1649250&tid=...
[3] Injunction upheld: http://yro.slashdot.org/article.pl?sid=04/07/23/1558219&tid=...
[4] Court's decision in English (pdf): http://www.jbb.de/judgment_dc_munich_gpl.pdf