A January 1951 advertisement from the Mutual of Omaha insurance company, courtesy of "v. valenti" on flickr, and boingboing.net:
The flickr link above provides a much larger image.
You gotta love this text in the header: "GAMMA RAYS VIRTUALLY HARMLESS OVER 7000 FT."
I assume this type of propaganda was initiated by the government to give the population a false sense of hope that an atomic attack was, in fact, survivable simply by following this "duck and cover" procedure. Mwa-ha-ha-ha-ha....