I’m surprised this has gone mostly unnoticed: two Japanese researchers wanted to port the Kernel-based Virtual Machine from Linux to Windows. In order to do that, they created a compatibility layer that allows you to run Linux drivers on Windows (in kernel, no less!). They gave a talk titled WinKVM: Windows Kernel-based Virtual Machine at last year’s KVM Forum, the KVM Developers Conference.
Source code is available here.
Given that Linux never removes support for any hardware from the kernel unless the driver breaks the kernel badly, the first use I can think of is obsolete hardware whose drivers no longer work on modern versions of Windows. That’s a often-encountered case in military and industrial environments: LAN Emulation cards, industrial devices using proprietary buses and proprietary cards to communicate to PCs, medical devices, etc. In those cases, the hardware generally outlives the computer that runs the software by decades (the software is no problem thanks to virtualization).
A second target “market” could be devices which do not have a Windows driver. Believe it or not, they exist in niche markets (and not so niche: the free Kinect driver was initially only available for Linux).