Zmeg and JB73 probably said it best. I have no doubt nukes would have been used. It would have been a very delicate situation with the Russians, seeing how they were right there. Something would have to be worked out fast.
I bet 'ol Ronnie could have done it too...gain the trust and cooperation of the Russians. The terrorists would not have lasted long, especially OBL.
It certainly would have been a strange moment in history if that happened, but it did happen to a certain extent during WWII, with Russia as allies.
A 9-11 attack when Reagan was president would have brought swift retaliation, and like JB said, would have been a worldwide confrontation, if not for the respect the Russians had for him and his advisors.
Les