John Carmack has revealed in his QuakeCon 2005 keynote that the source code for Quake III is about to become available under the terms of the GPL. Some time in the next week or so is the expected release date, but this has still to be confirmed. id is making moves to ensure that everything is licensed correctly before the release of the code.
Carmack also noted that he is looking forward to some day seeing somebody ship a commercial title, probably a budget game, under the GPL. He'd like, for example, someone use the Quake III source to complete their own shooter and ship it with the full source code included on the CD. Carmack expressed frustration with people who get overly protective about code that is largely built on the work of someone else.