In his QuakeCon 2005 keynote, John Carmack revealed that after some delays id is finally ready to release the complete Quake III source code for use under the terms of the GPL. He could not give an exact date, but was confident that this would occur "within a week". Currently id is making sure everything is licensed correctly and everything builds, and as soon as that is complete the whole package will be set free.
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.