"The land or water surface area directly below or above the point of the detonoation of a nuclear bomb" (definition by Webster New World), I guess this is what you mean by "Nagasaki, Hiroshima". Maybe this phrase was popularized from a military term during the WWII. But since Sept. every time it's used in news, it's always associated with the rubble of the twin towers. And I believe that's the context Bigrice heard it.

There is a second definition of Ground Zero, "the beginning, starting point, the most basic condition or level" (also in Webster).
I GUESS this is the meaning journalists are using this time around. The once soaring twin towers were reduced to altitude zero.

Please correct me if I still got it wrong.