A while back, someone here or on the Cinematography forum posted a comment about using "Fantasy Cloth" to soften backgrounds. I tried it, and found it potentially useful. However, the company seems to be phasing the stuff out. They no longer carry the Thunder gray, only ivory, blue, and purple.
Frankly, I think a much more useful solution to making backgrounds appear to be less in focus is to stretch a clean 8'x8' single or double scrim. They must be in perfect shape. They are also a whole lot more expensive. Currently you can buy the remaining Fantasy Cloth for $39 each in large rolls. Maybe the white would work better than the gray. They are at http://www.studiodynamics.com/category/Specials-46. (I found the gray somewhat aggressive in calling attention to itself.)
director of photography
San Francisco Bay Area
part-time instructor lighting/camera
Academy of Art University/Film and Video (grad school) http://www.RickWiseDP.com