Also Tim Marshall with enlightned equpiment has a cuben quilt that technincally is waterproof other than seams. He is designing the new ones to have a type of hand pump built into the fabric to help loft the down do to the non-breatheable nature of the cuben fiber
Interesting idea.. I've been thinking about doing something similar with sil -- I recall Ray Garlington doing something similar with his "Bag o'Feathers" that he made, he just put in a window of normal ripstop to help the down breathe (lofting, etc). Might help with condensation & whatnot, although given that i've never experienced it I am probably more paranoid than I have a right to be.