That Boeing language is a can of worms, IMHO. I wonder what it would be like to be a Captain for a company that has received approval to fly their NGs with frost on the designated areas of the wing upper surface. What do you do when a "knowledgeable" passenger makes a fuss in the cabin because they know the rule which says that the wing has to be clean? I think I'll stick to my Airbus and the clean wing concept.