Well, lets be honest here: It's not used in linux because Linus Torvalds would rather not have to learn anything new. It was used rather successfully in BeOS, and no one knows what Windows is written in, but I would be surprised if large chunks of it weren't C++.
And in the mainstream of OS development or embedded programming, you'll rarely find C++ *either*. It's considered too bloated and error-prone for use in the Linux kernel, and embedded platforms tend to lack a decent C++ compiler.
Also, C# was used in the Singularity project