I think it only depends on what its made of i.e Foam and Cardboard or plastic will be fine but if its live steel or something potentially dangerous to others.... you may have a problem (and under NO circumstances are they allowed ANYWHERE!)
Wow. Yeah those of us who have official capacities in cosplay at cons are I'm sure all going a little nuts trying to work with these laws- well certainly in trying to lay the rules down. I know there have been near incidents over here in NZ with air pistols but not obviously toy weapons.
I am currently working on a guideline for toy/prop weapons for Armageddon. I've always encouraged the making of these things for the creative side of things but now I'm having to look at the legal issues.
Thanks for the personal feedback silverfaction. that along with what I have been told by various organisers is enough to get me writing up sooner rather than later. Hate having to be in a position to say "thou must not" but it is a very real problem.