John Carmack announced the release of the engine source code for Return to Castle Wolfenstein and Wolfenstein: Enemy Territory today at QuakeCon. While S4NDMoD has no plans to work on the engine, the guys at ioQuake3 plan to work on both ET and RtCW. If you are interested in looking or modifying the source code, you can grab the RtCW Single or Multi player source code from the links below.
Return to Castle Wolfenstein MultiPlayer Engine Source Code (RTCW-MP-GPL.zip) (3.17 MB)
Return to Castle Wolfenstein SinglePlayer Engine Source Code (RTCW-SP-GPL.zip) (3.55 MB)