They planned for years, since they failed in '93, to take the Twin Towers down. They were like a fighter getting ready for the championship but training for a first round KO. If they had the means to kill 70,000 or 700,000 instead of 7,000 they would have used them.
OBL's goal is to foment Islamic revolution on a global scale. His best tool is propaganda. Look at the reaction in the muslim world to this attack. They are encouraged, emboldened and inspired to see that OBL, and by extension, they can reach out from a cave and hurt the great satan like this. If he could have destroyed the City instead of one symbol it seems to me that the reaction would have been exponetially more vehement.
OBL needs a massive, seemingly irrationally violent response by us to push the fence sitting moslem world over to his side. A nuclear attack would have likely provoked such a response.
No, I think they took their best shot on 9.11. What we face now is an IRA/PLO/HAMAS type of terror campaign on a smaller but wider scale. This campaign of car bombs, boat bombs in harbors, suicide bombers, etc will last for years or decades.
The real a-bomb threat is to our troops in Uzbekistan next spring. By then we will have 20,000 or more people on the ground in prep for the spring offensive. Pakistan has the means to strike Uzbekistan with atomic weapons. By spring it could be an OBL sympathetic finger on the trigger.