I believe the Saudi's have docks in several ports on the east coast, New York is one of them. Anyway, they wouldn't need to put a bomb in a shipping container, just inside any ship that ties up at a dock. How you gonna' stop that? You can't.
Maybe the only thing that gives certain countries the incentive to keep their cargo clean is the knowledge that they will cease to exist if any catastrophic event can be pinned on them
I guess we're just doomed then... and the terrorists won! Let's make $$$ in the meantime until we're all blown to kingdom come..