Recently Carmack announced that he recieved permission from his superiors to release the Doom 3 source code after Rage launches. Since Rage is expected to come out in October you can expect to see the Doom 3 source code come out sometime soon afterwards as well. 

Hopefully some crafty developers will take it upon themselves to release a source port for Doom 3 within the next coming years, or maybe some changes to the game could be made by developers already involved with a large Doom 3 project of some sort. A lot is possible when the source code is at hand, just look what it's done for the classic Doom we know and love!