I know that it is closed-source but I was wondering if MSFT ever thought about giving the a Linux/Unix hybrid kernel a try. Windows 2000 was great!
It's always interesting to speculate about these things, but the NT kernel is inseparably tied to the rest of the operating system. If you swapped the kernels, you would probably spend the bulk of your time writing compatibility layers in Linux/Unix to mimic NT kernel features.
Besides, I can't think of any value added by using a Linux kernel.
(and I'm a Linux user too, so this isn't a MS shill post)
The NT API is to strongly tied to many major applications and device drivers for that to be appropriate unless the Linux/Unix hybrid were to have API control layers to fix that (which would negate a lot of its functionality).
I think a better way would be for both Linux/Unix and Windows to get a common higher-level API, although I think this is adequately provided by WINE and .NET/Mono.