I\'m pretty sure if a nuke exploded anywhere, we\'d all be fairly "touched." Fortunately, we have the nuclear regulatory service which tracks all nuclear material within US boarders. It\'d be pretty difficult to even smuggle a nuke into the US without anyone knowing about it. Without proper shielding, the device gives off a halthy amount of radiation that makes the carrier detectable from miles away by radars or special scanning equipment, much more sophisticated then geiger counters, believe me.
However, there are non-nuclear explosives that are powerful enough to level anything within a mile of detonation, unfortunately for Terrorists, the military has been unsuccessful in streamlining this equipment and making them small enough to carry in suitcases. However, anyone can rent a van at Hertz...