Earlier today the Software Freedom Law Center (SFLC) filed the first ever copyright infringement lawsuit based on a(n alleged) violation of the GPL . Courts habitually scrutinize contracts for enforceability, and the GPL has never been tested by a court, so it's not a guarantee that the terms will hold up. (Disclaimer: I have some friends at SFLC, and I think they're quality people and good lawyers.)
The GPL, otherwise known as the GNU General Public License , allows people to copy, modify, and distribute licensed works only if they comply with the terms of the license, which require them to license their new code with the GPL and make it available to others to copy, modify, and distribute. Manufacturers of products incorporating GPL-licensed code have to make the source code public. The GPL uses copyright law to do this; any copying outside the scope of the license is copying without permission, i.e. infringement.
Monsoon Multimedia makes a Slingbox-like TV location-shifter, which incorporates a set of Unix utilities called BusyBox . BusyBox is licensed under the GPL, but Monsoon has kept the source code proprietary, while admitting that “much of the source is GPL and we should publish those sections which we have modified per the terms of GPL. A project is underway to pull this together.” (Hilariously, in the same post, Monsoon accuses forum users of violating its own EULA by reverse-engineering the code, which it is required by the GPL to make available for just such tinkering!)
If the facts alleged by SFLC are true, this is a pretty clear-cut case of GPL violation. The real question is how a court will react to an entity using copyright law to effectively limit the ability of downstream recipients to control copyright to derivative works.