Here are a couple of quick demos, illustrating how blurring a cubic environmental map can lead to a change in the perceived roughness of the surface of 3D rendered objects.
I created a series of HDR cube maps using NVIDIA’s CubeMapGen (currently hosted on Google Code). Starting with the Debevec light probes, I applied a Gaussian blur with increasing kernel size (10°, 20°, 30°, 40°, and 50°), creating 6 cube maps (one for each blur). In the videos, the cube maps have increasing blur from left-to-right, top-to-bottom. Note that I did not tone-map or account for changes in overall exposure (so the specular reflections can appear blown-out, especially for the higher blurs). After the break, you can see the effect using different light probes (and different shapes).