Unreal engine was built around Glide for 3d acceleration, then it was patched up bit by bit to run nicely in D3D, the UT engine superceded the Unreal engine and was pretty good in D3D mode but could stand some improvement.
I think the hope would be that Unreal II is built around D3D from the ground up for 3d acceleration.
It doesn't matter whether or not NVIDIA is the only 3d chipset manufacturer (which they aren't anyways), software companies must contend with the installed user base which includes everything from Integrated Intel 815 video, to ATI chipsets, to users of Voodoo3s, etc.
Just look at Quake 3, it was built around OpenGL but still had to support a similar range of video cards.