The source code for Doom 3 (id Tech 4) has been released today. John Carmack has talked about releasing it at this years QuakeCon, honoring the long time id Sofware tradition of open-sourcing their games, dating back to Quake 1. There was some delay in the release, due to id Software not being able to include a patented algorithm known as Carmack's Reverse.
This release has many positive implications for the modding communities based around id Tech 4 games, including that of Quake 4, and for those who simply want to satisfy their own curiosity.

Carmac's tweeter: