The RtCW source code can be downloaded from FileShack, with separate multiplayer and singleplayer components, as can the Enemy Territory source code. All are licensed under version 3 of the GNU General Public License.
id's chief technical wizard John Carmack had promised that the RtCW and Enemy Territory source code would be released soon after QuakeCon 2009. He explained yesterday that they were so delayed as he had been too busy to clear them with legal.
Carmack explained that these things aren't always as clear cut as the community might think, as it opens the company up for liability if someone somewhere used a bit of code that wasn't original or they didn't have the rights to use. Despite that, Bethesda decided to okay the source releases.
This is the latest in a long line of source releases from id, who has so far shared the source code for all its engines from Wolfenstein 3D up to Quake 3: Arena. Carmack teased that id would start looking into releasing the Doom 3 source after Rage ships.