Everyone keeps joking about Bic pens and pencils and other such items.
Ceramic knives are generally extremely sharp, and readily available.
As an example, the Ming Tsai line of ceramic kitchen knives can be found at your local mall. They're a bit more expensive than your standard steak and butcher knives, but as we've already been shown, the bad guys have deep pockets.
This is a lot more serious than we might think. (Not to say that the potential terrorist will probably be the recipient of a severe beatdown, but this is not the sort of throwaway alert that we've become accustomed to.)