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.