I've heard that too, but seen no actual sources. I have been getting messages all evening saying along the lines of 'I know someone who knows someone and its cancelled'
I personally don't think the RAF displays will happen which will in turn mean the show being cancelled. I am also getting a lot of messages asking why they haven't put out a statement yet. I'm guessing it takes time to co-ordinate with all the various groups involved in the show before a statement can be made.
I don't expect the show to happen now. But I think the best thing to do is wait for official word.
I actually think notifying the traders first is a good idea, keeping up good relations etc as they are paying to have their pitch there. They would be pretty annoyed to find out via FB rather than being notified.