Just took my bottle conditioned and (well) carbonated pumpkin ale (I live and brew at sea level) up to 2700 feet today for some mountain fun. It is at about 2.7 volumes of CO2 here at sea level. It did seem more foam-y at the mild elevation at which we were. I went looking up info on this phenomenon today and found this thread. I was wondering if it was just me, but it seems that there is some science behind the effect we noticed. Just thought I would put forth some experiential evidence for those that may be looking for the same information. Bottle low, drank high = more carbonation- perceived or otherwise!