I have an Integral Designs Unishelter Exp in the Event fabric. Sometimes there's a little condensation, but it's not a big issue because the bivy has a wire hoop over the head area and my sleeping bags have water resistant covers. I do wonder if condensation would be a bigger issue if the bivy didn't have the hoop over the head area and the tunnel vent. I love those tunnel vents - I have an Integral Designs mountaineering tent with two tunnel vents.
http://www.integraldesigns.com/product_detail.cfm?id=776&CFID=2253389&CFTOKEN=97148252
My first bivy was an Integral Designs Salathe Bivy and I don't recall condensation being a problem with that bag at all - it is made of their TegralTex fabric, which is similar to the fabric used on Bibler Tents (not the lightweight Black Diamond ones).
