Chaps, here we have the classic pprune 'x' answers and 'x+1' different opinions! I tried to condense in reply #3 what my flying manual said for the 747 and 737. Thin frost on the fuselage is allowed. I know nothing about allowing frost on the upper wing surface. I guess everyone is right in their own way, but the only 'right' answer for <it.pilot737> is what's written in his flying manual, and a healthy dose of respect for the problem.
So many aeroplanes have piled in because of ice, and still do, that I find it strange how the rules change and if frost is now allowed on the upper wing surface. But I always believe in asking the people who know. What we need is a combined rule setting meeting of Air Canada, Air Alaska, SAS and Aeroflot to set the guidelines, and keep the politicians and office flyers out of it!