Carl Irwin explains how to achieve motion blur when rendering OpenGL from the viewport.
This is a follow up to a previous tutorial dealing [included below, Bart]with the creation of a 'high pass filter' mask for the purpose of compositing motion blur on footage that has only 1 sample in render (as is the case with OpenGL renders). This tutorial deals with the creation of an authentic motion blur using an ultra low resolution multiple sample render pass as the basis for the blur mask and overlay as in the previous tutorial. This technique achieves a far more accurate result in very little time by avoiding the render of additional samples at full resolution.
Blender 2.7* Create 'Motion Blur' from Thin Air (VSE Tutorial)