This morning I wrote about What Should Linux Gamers Expect From QuakeCon 2011? In there, I mentioned the id Tech 4 engine source release was likely based upon past QuakeCon traditions and John Carmack previously promising the code's released upon the release of the RAGE game with id Tech 5.
This is indeed the case and just moments ago during John Carmack's QuakeCon 2011 keynote he confirmed the release this year of Doom 3.
The source-code to Doom 3 is definitely to be released this year, Zenimax (the company that acquired id Software) has been supportive of the process, and he has challenged other game developers to release old source-code too. Carmack says that releasing the code really helps the development community.
Rage is set to be released for the PC in October, so the Doom 3 source release will likely be at some point thereafter. Carmack has yet to comment on the Linux state of the id Tech 5 engine or a Rage Linux client, which is suspected at some point but likely not on launch-day.
Stay tuned for more details. Update: There's also some bad news from id Software.