Some interesting links regarding this topic.
http://www.fsf.org/licensing/licenses/gpl-faq.html
Does the GPL require that source code of modified versions be posted to the public?
http://www.fsf.org/licensing/license...cePostedPublic
If I use a piece of software that has been obtained under the GNU GPL, am I allowed to modify the original code into a new program, then distribute and sell that new program commercially?
http://www.fsf.org/licensing/license...PLCommercially
Can I release a modified version of a GPL-covered program in binary form only?
http://www.fsf.org/licensing/license...fiedJustBinary
If I add a module to a GPL-covered program, do I have to use the GPL as the license for my module?
http://www.fsf.org/licensing/license...LModuleLicense
If a program released under the GPL uses plug-ins, what are the requirements for the licenses of a plug-in?
http://www.fsf.org/licensing/license...#GPLAndPlugins
What should I do if I discover a possible violation of the GPL?
http://www.fsf.org/licensing/license...rtingViolation