I know of at least 2 events that will, if and when they occur, have a dramatic impact on the world economy - which would probably make Sep 11 pale in comparison.
1.
Peak Oil: I had thought this was not going to happen for a few decades now, until I watched British Petroleum's Chief Executive suggest on BBC that it would happen by the year 2020.
2. Prime factorization: This, unlike peak oil, could happen any time. All it takes is for someone to come up with
an algorithm in P to decompose any number into its prime factors.
Given the drastic effect a prime factorization algorithm would have on the US economy, I wonder if anyone who cracks the problem will be designated a "Math Terrorist". Wouldn't that be cool? :)