The Xbox 360 will probably will be id's primary development platform. As it is right now, we would get the game up on the 360. When I would do major hack-and-slash architectural changes it was back on the PC, but it's looking like the Xbox 360 will be our target. All of our tools are on the PC, and we're maintaining the game running on the PC, but probably all of our gameplay development and testing will be done on the Xbox 360. It's a really sweet development system.And with that, I will not hold out hope to see any future id Software games on GNU/Linux, as has happened with every previous game. Maybe we'll see a GPLed release years after a game's release, but I can't see id Software justifying the money to support smaller platforms when they're heavily married to Microsoft like that.
Maybe Carmack is really keeping it crossplatform. Certainly when he says "we intend to release [unannounced next game] on PlayStation 3 simultaneously", it suggests as much. But it appears that the whole focus has shifted for them. It used to be Windows first, with MacOS and GNU/Linux second and distant third, respectively, and then a console port if an opportunity arose. Now, however, it's console first, with Windows a trailing second, and that pushes MacOS and GNU/Linux even further to the back of the line.
My friend Dustin tells me I've just been living in a bubble, that this is old news. He points me to a Shacknews story from last year saying "the Xbox 360 is the primary development platform for the [new Wolfenstein] game and it's based on new technology" and this quote from Carmack's X05 keynote speech: "XBox360 is now id's primary platform for development." I guess reality was destined to visit me eventually.
Finally, to vent one last bit, I've always been a little disappointed that Carmack didn't throw himself into the OpenGL driver development work on GNU/Linux again after DooM 3 launched. Back in 2001 LinuxGames reported:
Also, should other drivers not be up to par when DOOM  is nearing completion, he may attempt to work on an open source driver project similar to his stint on Utah-GLX.Sure, he left himself an out, saying "may", but from the guy who GPLed DooM, Quake, Quake ][, and Quake ]|[ Arena, I expected more. Next time, I'll know better.
This doesn't preclude anything, id still has a programmer who mostly works solely on Linux ports. The games are all still OpenGL currently (SDL for handling of window/input on Linux at least), with OpenAL. Unless they decide to dump openal/opengl and to a lesser extent, SDL, it isn't going away as a possibility.
You know how easy it is to stop GNU/Linux development? Easy: "TTimo, we're letting you go."