However, all that said, I'm not aware of any instance where the GPL has been tested in court (or even "out of court" as they say) against a company accused of taking GPL code and developing closed source products from it.
Read "Enforcing The GPL" I and II at http://emoglen.law.columbia.edu/.
Apparently the GPL has been tested out of court several times, and Moglen claims it has never been tested *in* court precisely because it is so watertight.
GPL has the nicely recursive feature of being considerably less restrictive that the normal license agreements from which the rest of the software industry makes its money, so if anyone managed to get GPL challenged, all such licenses would be suspect. As the only group likely to be so motivated is precisely that same software industry, this won't be happening any time soon. :)