XNA doesn't even support DX10, let alone DX11, so there's no way it's going to help C# overthrow C++ anytime soon (or be used in BF3). As for the XBox argument, many large companies use middleware to generate the API-specific rendering code, so they don't actually touch it themselves. It's fine for hobbyists wanting to get a start on creating full applications easily while they get more comfortable with implementing the finer details themselves, but not much else.

C# is great for tools and prototyping (and business applications, obviously), but you have to be a serious hack to require it to do production code involving visualization. I use OpenGL (mainly via OpenSceneGraph) and DirectX for scientific visualizations in my job.