Is bridging a GPL application with a non-GPL application legal?
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.