In Cycles how come I cannot see the motion blur in the 3D viewport when in render shading mode?
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Look here -> http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Integrator under Motion Blur ... " Viewport rendering currently will not show motion blur." Not yet but maybe one day ;)
From a technical perspective I'd say - viewport is viewport (and has nothing to do with shutter, focal length, aperture etc.) - If you hit F12 you "jump" to your camera in your scene which has all those specs ... that seems quite comprehensible.
That's because the render process and the viewport rendering are two different things (from a programmers perspective) - that's all I can say ... maybe that will help you What are the technical differences between F12 render and viewport preview?
I guess this isn't implemented yet because it would slow down the viewport even more. To see the effect you could alternatively test render a frame. This function regards compositing rather than what the viewport is intended for.
In order to calculate motion blur you need multiple samples (frames) of the image at different times. Some algorithms use the optical flow if you decide to have a preview of 20 samples the image it would take 20 renders plus motion blur to create the viewports image.
A nice overview of common algorithms you find Motion Blur Rendering: State of the Art