This is cool news. I really liked Doom 3 when it first came out, it was one of those games I needed to play with the lights on since something was always jumping out and scaring the crap out of me. If you are a fan of Doom 3 id's John Carmack has now offered the source code up to the open source fans out there.
There are a few catches to the open source release though. The content has no game data and that data is covered by the games original end user license agreement. The code also has the rendering of stencil shadows functionality removed that used depth fail. This is the part of the code that is thought to possibly infringe on a patent Creative Labs holds.
The code has a work around in place that lets it get around that patent issue. Carmack has apparently said in the past that the work around needed only four lines of code and two existing lines were changed.