As noted in Timothee Besset's .plan, and as promised during QuakeCon, the source code for Quake III Arena (direct link) is now available on the id FTP server. The 5.5 MB download is mirrored on 3D Gamers, FileFront, GameSpot DLX (registration required), and Worthplaying. Word is:

Quake III Arena source code released under GPL
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We're done packaging up the Quake III Arena source code for a GPL release:

af8aaac46cc82035e1cba632b1a16e03 quake3-1.32b-source.zip
ftp://ftp.idsoftware.com/idstuff/source/quake3-1.32b-source.zip

note that a build of Q3Radiant 200f is included under GPL as well

some D3 SDK things:
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I've uploaded an XCode project file [1] to make it easier to build
the gamecode on Mac. There is no SDK source update needed at
this point.

[1] ftp://ftp.idsoftware.com/idstuff/doom3/source/mac