Global firms and markets don't want just one global city: they want a network. The network of global cities is a kind of strategic infrastructure for global firms and markets. Today there are about 40 such cities, with a few at the top (London, NY, Tokyo, Paris, Frankfurt) and then two or three other layers.
Much attention has gone to the homogenising of cities due to globalisation. This is really about the making of a state-of- the-art environment for offices and high-income households. But actually what really matters is the specific economic history and specialised difference of each city.
A big question we need to examine is whether terrorism can alter these patterns whereby the global network of global cities is a key infrastructure for economic globalisation (and increasingly for the formation of global civil society). It certainly will push any firm that does not need to be in the centre of a global city, to leave. In that regard 11 September in NYC is a sort of natural experiment. We can say that it would eliminate any inertia that might have kept firms in Manhattan.
The patterns are interesting. Initially many firms left from the downtown area and relocated to New Jersey and Connecticut. But after a while quite a high share of these returned - not in full, but critical parts of their offices. Many went to midtown Manhattan, and may or not move downtown.
It does suggest that the type of agglomeration of people, firms, talents, knowledge that you have in a major city is still indispensable.