London Bridge is falling down

I'm addicted to Fergie's newest (well, new-ish) hit "London Bridge". Although, hearing it played over the radio five times yesterday wears on the brain a bit. But I still love it.

However, can someone explain to me exactly what it means for one's "Londy, London Bridge" to wanna go down?

I consider myself somehwat hip, but I'm having a hard time putting my finger on exactly what this could mean. Does it refer to her britches? Or maybe some recent dental work? Somehow, that childhood song will never quite be the same...