My understanding was the DirectX thing is mostly to do with XBox, although there has been uncertainty as to whether the primary development is now DirectX as apposed to what it has always been, OpenGL. this could all be speculation, as there has been little info released, except for what I said above about a comment made by John Carmack. They have stated that Rage, their new IP developed on id tech5, will be available on Console (PS, XBox), PC (Windows) and Mac, which of course means OpenGL, it's not like they are dumping it or anything, but nothing actually mentioned about Linux.
I also remember at that same time as the comment I mentioned above, John Carmack had some non to flattering things to say about Linux and open source in general, can't remember the exact details now, but I think it was something to do with the zealotry in the Linux/Open source community, and may have also been to do with the fragmentation of Linux in general. I do remember at the time I read it, it made me a little sad, and unsure of the future of id games and Linux.
It also seems that JC has become more and more impressed with DirectX over the years, so much so that he also made a comment along the lines of, that there is no longer any reason to choose OpenGL over DirectX
Although the above is to do with Rage, and not Doom4 as such, they are both being developed with id tech5. Also, this is just what I have read. I read, watch everything concerning id software and id software games that comes my way, as in the gaming world, nothing else really interests me. So make of it what you will.
Myself, I believe that Doom4, following the tradition, will be available for Linux, I am just not, as John Carmack said, taking it for granted.
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On a personal note, this is all a moot point for me personally, as I have been offered a place in a project which will require me to use specific hardware and proprietary software/file formats, namely ProTools, which will not function under Linux, this is so I will have compatibility with the other band members, and that projects can be taken from home-rehearsal-studio, while maintaining full compatibility at every step. It is an offer I simply can't refuse, I have purchased
THIS, which should be arriving tomorrow, I will then have to reinstall Windows, so my Linux time seems to be at an end.
It's not all bad, as I will be certain that Doom4 will work for me

, plus being an avid game mapper/modder, I will also be able to get back into that when the time comes, something which I would not have been able to do in Linux, regardless of whether there was a Linux release of Doom4 or not, as the Modding/mapping tools are win32 only, from Doom3 onwards (Quake4, ETQW) with the built in editor and tools, it has been this way, and apparently there are no plans for that to change, no plans to make the tools Linux compatible. This is something I have missed under Linux, while I still have been able to continue dabbling with Quake, Quake2 and Quake3, the later games are just not possible with Linux.

Fare Thee Well Good people
Have one on me