Don't ask why or how this came up, but with the talk about Windows redux (1 , 2 , 3 ), I had an interesting thought: imagine Windows being rewritten to run on the Linux kernel. Don't get me wrong, I don't think this would ever happen, but could you imagine? The OS wars would almost be completely naught. You'd have the Linux family, which are all having their own internal battles today; Mac, which is based on Unix; and Windows, the mack-daddy (based on usage), would take the same approach Mac did with OSX, whether based on Linux or Unix, the results would be the same.
Linux advocates would feel like they've won, in a sense, because Linux would then become the most used kernel by far; however, we all know the cost model wouldn't change, so then the only difference would be between the user experience and proprietary software. With this, Microsoft would have to bring .NET to the Linux world, which would completely consume the major portion of Mono. By this time, I imagine the majority of the Windows-specific features and core applications (i.e. Office) would be implemented in .NET, which is what I believe they're moving towards, anyway -- not 100% of the code, but as much as makes sense at whatever pace makes sense. Java would see an even more drastic decrease in usage because .NET would see more "multiplatform" benefits. Honestly, there's so much you could speculate with such an extreme move. All you can really do is laugh at the idea.