Our eyes are constantly taking visual data from our surroundings, the data our eyes feed to our brains is not sampled. This is a difficult effect to achieve on the computer. There are algorithms that allow a computer to achieve real motion blur, but these tend to be really slow if your using a large number of polygons. Much faster ways of approximating motion blur exist. This method is called distributed ray-tracing.
Problems exist in the distributed ray-tracing method as well.
So, Impulse would have to over-haul Imagine to implement motion blur. If you look at versions 2.0, 2.9, 3.0 and soon 3.1, you'll notice that almost all the new features are add-ons and really aren't integrated into imagine as a whole. It's great that Impulse planned far enough ahead to allow Imagine to be so modular that these new features are just added-on. Unfortunately, I really don't think Imagine's modularity goes far enough to allow an easy addition of motion blur. I sincerely doubt that we will see motion blur in Imagine.