The implementation is a one or two day thing, for example i have seen people implementing the Oculus SDK only in a couple of hours (the 3D support at lest), it just controls the camera. But if you want to make a game around the Oculus hardware that is a different story.
If Frictional Games wanted they could do it, the problem is that they don't have a developer kit (nor the desire to get one) and because of all the success of this hardware the new orders are taking a very long time to arrive so it could be months before FG get's their kit (could be less if they make some kind of deal with the oculus guys). There's also the problem of "you only know what it is when you experience it" so if you come with the idea that this is just like the 3D Cinema then you don't have the desire to get it. But let me tell you this tech is nothing like you have seen, it is completely different to the 3D cinema, on it the 3D effect is just like it is on real life (theres no ghosting or blurring like on the 3D trick used by Cinema 3D), theres a real sense of scale in the virtual world, totally different to what you experience on you monitor, and i can go on and on, but has always you will only know what it truly is when you experience it for real.