You can read more about the discussion (and join in) at the thread Encryption of Joomla! extensions is a violation of the GPL
[Update]
I have had a bunch of emails, many saying thanks for drawing attention to this, and a few questioning my interpretation of whether the license has changed.
I’d just like to explain my perspective here. Up until 13 days ago, the license of Joomla 1.5 had two important paragraphs at the end. These clearly allowed extensions to be licensed as something other than GPL. These paragraphs were removed. If I download a nightly build, the license is different from what it was before.
The real issue at hand is clearer. As the thread at joomla.org has now been changed to "Can Joomla! extensions be released under non-GPL compatible licenses?"
Of course this is just my opinion. I heartily recommend you head over to the discussion and read and make your own determination!