It would take a very large nuclear explosion to cause such a tsunami. It's probably not too likely terrorists would conspire to do this when detonating one on land would have a much more terrorizing effect. Meteors, on the other hand could wipe out coastal cities throughout the Atlantic or Pacific depending on where they hit.
This is a pretty good site about tsunamis in general: http://www.prh.noaa.gov/itic/library/about_tsu/faqs.html
This link shows the (small) possibility of a larger tsunami in the Atlantic:
http://www.newsmedianews.com/tsunami.htm