As the US and the UN seem unable to contain Iran's nuclear ambitions, we have to prepare ourselves for an unsettling future. Unlike the Soviets and Chinese, we can't count on the Iranians or other Islamic fundies to whom they might pass a nuclear device to behave rationally. MAD is not a viable strategy when your enemy welcomes death as the way to be with God.
Domestically, a nuclear Iran and proliferation in the Muslim world may politically destroy the left wing of the Democrat party, especially if a nuke gets used.
There's some excellent discussion of the issue here. I think I'll order a copy of this, as well.