More on NEST:
http://www.thebulletin.org/issues/2002/ma02/ma02richelson.html It also sounds like they wouldn't make the decision to deploy them lightly, which means that the fact that they were all over several cities, the threat was absolutely real, and most likely still is.
This article was written in March 2002, so it doesn't comment on the recent threat and NEST deployment.
"Since NEST’s creation, about 100 threats involving alleged nuclear devices or radioactivity have come to its attention. At least a dozen, and possibly more than twice that number, have resulted in deployment of NEST personnel. NEST, in general, will not confirm or deny when or whether it has deployed to a particular city or region. However, it has been reported that between 1975 and 1981 NEST personnel were sent to investigate threats in Boston, Los Angeles, Spokane, Pittsburgh, New York, Sacramento, Tennessee, and Reno (Time, January 8, 1996; Washington Post, June 21, 1983).
The threat to New York came in July 1975 when terrorists claimed, “We have successfully designed and built an atomic bomb. It is somewhere on Manhattan Island. We refer you to the accompanying drawing in one-eighth scale. We have enough plutonium and explosives for the bomb to function. The device will be used at 6:00 p.m. July 10 unless our demands are met.” As reported in the New York Times Magazine, the key demand involved $30 million in small bills.
NEST was impressed by the drawing. According to one account, it was sophisticated, precise, and “made by someone with more than a passing acquaintance with nuclear physics.” But that did not lead the United States to part with real money. A dummy ransom package was left at the drop site in Northampton, Massachusetts, and FBI agents waited for someone to claim it. Nobody showed up and there was no further communication from the extortionists."