Going the other way like Microsoft does is more interesting.
One of the biggest issues with OpenGL is that you can get shaders that won't run in bounded time. You can see this with a number of games in Flash, or natively in OpenGL, when run on a Mac. If the shader doesn't exit, it eats a channel, and there are a limited number of channels, and once they are gone, the renderer, which is also used for the desktop, basically crashes. There are nice system log messages from the video driver about it, but besically everything ends up restarted, which is pretty useless.
FWIW: this accounts for the majority of system instabilities in the card specific portions of both Mac OS X and Linux render pipelines.
DirectX doesn't allow things to run in unbounded time in its OpenGL to DirectX translator; instead, it loop unrolls shaders, and if it can't do that such that they run linearly, and therefore in bounded time, it omits them from the render. So you might not get distance blurring, haze effects, fog effects, rain effect, and so on, but at least the thing doesn't crash, and if the person porting the code to the Windows platform cares about these things, they fix the code so that it'll run using DirectX. Usually, this reduced the perceived "quality" of the final render, but you get at least a crude version of your effect back.
The other thing DirectX does is, in the video driver, keep a reserve channel for sending commands to the video hardware; the common reason for this is in-band signaling to comply with the DirectX 9 requirement that the video hardware be capable of being reset, without rebooting the system, such that a video card hang doesn't necessitate a reboot.
While a DirectX to OpenGL translation layer is a nifty idea (I lobbied very hard for a FreeBSD emulator for Linux, rather than a Linux emulator for FreeBSD so that developers would target FreeBSD rather than Linux as their development platform), I don't think that as long as the OpenGL shader looping issues don't also get addressed at the translation layer that translating from DirectX to OpenGL will be in anyway superior to translating from OpenGL to DirectX.
So basically, it's nice they released this, but the code is of little practical use in the real world, since there are features that will get lost in translation.