The GPL is a license, not a contract.

Low Zhen Lin <[email protected]> Tue, 16 Dec 2003 11:58:25 +0800

Newsgroups	gmane.linux.zynot.devel
Message-ID	<[email protected]>

http://www.groklaw.net/article.php?story=20031214210634851

Basically - no particular part of the GPL forces the code to be open; 
rather, it is a two-way choice: open the code, or do not distribute the 
derivative. Which is more or less the same as forcing - but we're 
playing semantics.

Printable version: 
http://jupiter.zynot.org/~zllow/GPLIsLicenseNotContract.pdf

Followups to [email protected] only, please.

- Zhen Lin