The Joomla core team have published an announcement regarding the GPL Licensing for Joomla 1.5 moving forward.

For more information read the open source matters article at the offical web site.

You can also find more information at the following sites regarding the announcement.

1. Joomla forum topic GPL Questions Continued, Developer related plus forum lounge area
2. Joomla 3rd party developer group site The Joomla Commercial Developer’s alliance