Diz - I ran into this and still do. If there is too much fast air movement, then the plants transpires alot more, and the nutes build up in the leafs. And you get the curl. One of my MPB's has this problem constantly because of a/c placement. Even the floor fans can do it if the air they are moving come in direct contact with the plant. I have run into a poorly placed floor fans pointing up and hitting some part of the plant causing this.
You are absolutely right about it the plant getting direct air from a fan, its not a good idea unless its very very gentle.
The first pic is a close up of the fan curl I get. The second pic shows the side of the 1 MPN that gets direct a/c, and its actually pretty gentle. The last pic is a shot of the girl next door..