Only reason given so far is that the US Embassy in Paris contacted Air France and I would imagine the French government re: cancellation of these flights today and tomorrow. Embassy was contacted by Fox News but they would not comment.
I remember one news article saying that among all the terror chatter and informants etc we even had flight numbers of planes that AQ was interested in. I can't remember where I saw that.
I think the next hijackings - if there are any - might be done near the end of the flight, to give us less time to respond.