Amy Stephen over at Open Source Community has put together a good summary for how differing open source CMS projects have interpreted the impact the GPL has on third-party extensions/modules/plugins/add-ons. Movement in the Joomla community ensuring GPL compliance for extensions is what prompted her comparisons of license interpretation between Drupal, Joomla, Plone, Typo3, Wordpress, and XOOPS.
Joomla!'s announcement from June 15, 2007 that began Joomla! is moving to ensure the future of the project by committing to compliance with the GNU/GPL license was a bit shocking to many accustomed to the Mambo proprietary extension licensing exception.
Sometimes I wish I was a lawyer because it really is difficult to know who is right and who is wrong in their interpretation of the GPL. Probably the most confusing interpretation is in the area of bridges where a third-party module is used to connect a GPL application with a non-GPL application. The significant impact of a strict interpretation of the GPL license can be clearly seen though Simple Machine's announcement of a SMF Bridge for Joomla! being discontinued due to the license interpretation change. If you read the correspondence between SMF's developers and the Freedom Software Foundation representatives you come to the conclusion that even though the bridge is GPL you could still have legal issues if both applications being bridged are not under the GPL.
No. The glue script would ultimately create a single work, derived from both the original scripts, and you would need to follow the terms of all those licenses to create it. Combining the first script with the second this way would violate its exception-free GPL.
Licensing Compliance Engineer, Free Software Foundation
Anyone else confused? I know I am (which isn't difficult for me). If a GPL project doesn't offer an exception...I wonder how many third-party modules offered as bridges between a GPL applications and non-GPL application are in jeporady for that project? I also wonder how open source CMS projects address those exceptions to the GPL license? While projects may address those exceptions within their community or their official sites, I wonder how many actually include a copy of those exceptions on file within the distributed package?