John Carmack has stated in an interview with Game Informer that their brand new engine will not offer DX10 support and this will probably never happen, unless he is strongly compelled to do so by "circumstances".

Even more, he does not seem to be overexcited about the prospect of DX10 mostly because it is based on D3D and their best work has always been done in OpenGL. More of this fascinating tech talk can be read in this interview excerpt:

"GI: Will this engine support any DX10 features?
Carmack: No, not currently. We're not expecting to. We're not sure if we're going to be a Vista title or not. There will be some support benefits by being Vista only. It depends when we get the game done what the adoption has been. But it's a OpenGL title on the PC and Mac right now, obviously D3D on the 360, and the PS3 it's kind of an in between where it's Open GLES but we do a lot of direct command buffer writing there. If necessary we can move the PC version over to DX10, but there's not much strong pull for us to do that. All of the toolset is in OpenGL, I wouldn't want to convert everything over.

GI: You didn't seem to hot on DX10 or Vista at CES.
Carmack: Microsoft has done a great job with all this stuff. I mean, I honestly think that DX9 with how it's implemented on the 360 is a clearer and more open API than OpenGL is. It doesn't hide the state.

That's sort of the Microsoft way. They start off with a piece of crap, and then over a number of versions taking a lot of people with them over the painful route they eventually get to something that's better than what they are competing against. It's a valid strategic direction. I think they've come out at the end with a good platform and a good product. Some of the DX10 stuff I don't think there's going to be huge draws for the features there, but a lot of what they've done with the structuring of the API I think are still positive things to do there. I think they have a good team with solid engineering there."

John Carmack hasn't been the most beloved developer in the industry because of the delay he imposed on Doom 3 and other id Software games. Nonetheless, his word weighs heavy in the gaming community and when he says that right now DirectX 10 on Vista is a piece of crap and that it will take some time for Microsoft to polish it, people tend to believe him. I certainly do.