I dont know about this - expanded polystyrene does not stand up well to heavy sunlight and I've seen rodents chew through it to make nests. Maybe this stuff is too dense for the rats and the coating protects it from uv degradation. Still not sure how this is Green (other than pure marketing). chemicals for styrene are not eco friendly. On the other hand, it is sculpt-able and you could make great non-linear walls.
As for Rapid 3d - there are several people working on 3D printing of buildings: http://www.contourcrafting.org/ (outgrowth of USC research)
and Enrico Dini of D-Shape: http://d-shape.com/
Interview from Shapeways ---
Sand, concrete, hmm.. Maybe we can use some lava! Although that might cause problems pouring around the pipes.