OK, well 0.92 is out and you can try playing with the motion blur yourself. Motion blur is achieved through an algorithm called temporal antialiasing: this means the framerate is, say, increased by 10x (so it might be running at 750fps). Then every time it draws the screen, it has 10 frames which it blends together equally to make an image that appears blurred according to how fast objects are moving.
To make this work with the mouse, it linearly interpolates the mouse position over these sub-frames because the mouse doesn't update that quickly. This explains why you get straight lines, and it also explains why if you move the mouse or an object extremely quickly, you see it stepping along. This still looks smoother than without motion blur, in my opinion, because you get a sense of the motion as opposed to the object displaying in a sequence of positions. Temporal anti-aliasing isn't perfect - if you don't like what it does you can just turn it off - but I think it really improves the motion and perceived smoothness of most games.