We have done some tests using this technique, but the result is not optimal. Most of times, the point where 2D and 3D objects blend is too noticeable, specially in animations.
You can simulate it using two Forest objects with opposite Camera->Density Falloff curves. The density of 3D objects decrease with the distance, while the 2D density increase. I've attached a sample scene.
Btw, what render engine do you use ?