Running around carrying nuclear weapons is not something that is terribly easily, especially if you want to have many people around the country carrying them. There is definitely going to be more terrorism, but I'm more worried about biological terrorism than nuclear.
In any case, this act of terrorism will not cripple the U.S. (in the long term, it may serve to strengthen it), and other acts of terrorism will not cripple it. The most likely next stage of terrorist attacks that I think we are likely to see are smaller scale ones. This one was probably intended to send a message and get our attention, which it did.