What benefit does Iran achieve by detonating a nuclear device against the West? At the end of the day, the terrorists want something from their Jihad. Osama said to attack and kill the Jews and Crusaders. Mahmoud said that Israel should be wiped off the map. While both political firecrackers, one is definately more menacing than the other. Mahmoud could be talkiing about relocating Israel (to the US, Canada, or EU) as he has before, but Osama clearly wants to fight.
I agree that we should not drop our guard, but I don't see any benefit for the Iranians to pre-empt and launch a nuclear assault on the US or the West. They couldn't annihilate us, and we certainly would annihilate them. They aren't going to convince us to convert to Islam, by dropping the bomb. In Al Queda's case, they wish to die as Martyrs (they are all going right to heaven when killing themselves with us). If the Heavens Gate folks in San Diego thought that the spaceship would only pick them up, if they killed Muslims, Jews, or any other race or culture when they offed themselves, you can bet that there would have been more collateral damage.
Even if the letter is a "final invitation to convert" it does not necessarily follow that the Iranians would attack us openly.
The modus operandi of the Iranian islamists to date has been to use terrorist surrogates to attack American forces(Beirut Marine barracks, Khobar towers barracks, etc.) where the ultimate command and control is hidden, and not to take open credit for these attacks.
It need not at all be the case that the letter and any associated clerical rulings are intended to be the basis for a nuclear strike, it could be that what is intended is a biological weapons attack, suicide bombers or some other terrorist operation.
You're thinking like a civilized Westerner who's benefitted from centuries of Christian culture.
What if, however, you were convinced that God had put you in a position to make The Book of Revelation come true? And that, no matter what happened, God would bring you to victory?
That's probably the sort of mindset we're dealing with, with this guy. He's probably got the same sort of hold on his people, that Hitler had on his.