There is certainly no plans for a commercially supported linux version of Rage, but there will very likely be a linux executable made available. It isn't running at the moment, but we have had it compiled in the past. Running on additional platforms usually provides some code quality advantages, and it really only takes one interested programmer to make it happen.
The PC version is still OpenGL, but it is possible that could change before release. The actual API code is not very large, and the vertex / fragment code can be easily translated between cg/hlsl/glsl as necessary. I am going to at least consider OpenGL 3.0 as a target, if Nvidia, ATI, and Intel all have decent support. There really won't be any performance difference between GL 2.0 / GL 3.0 / D3D, so the api decision will be based on secondary factors, of which inertia is one.