From the martial arts instructor perspective:
Plastic and metal knoves are equally dangerous. A credit card, a piece of plastic "glass" with sharp edges, can of Coke which had been torn in two, your keys, the clasp on the metal bracelet on your Rolex, eyeglasses, your hands if you position them in the right places (i.e. eyes, throat)- all these things can be lethal with the right training and a certain frame of mind.
I am not sure how practical it is to ban everything that can be considered dangerous.
It seems to me the responses of the airline industry to the Sep 11 events are not necessarily designed to make things better/safer, but to change the perception and make public and air crews feel like something is being done.
However, the most importange change has already happened: awareness and alertness have increased, among the crew and among the passengers.
With a bit of training and knowing that the consequence of doing nothing is likely to be death, most people are sufficiently motivated to participate in the mid-air problem solving, IMHO.
David