The term has been in use for centuries. Until (relatively) recently, it referred to the dead. References to the term being used for the majority of voters go back to the 1800s. Nixon got his use of it from John F. Kennedy's book Profiles in Courage,
"Some of them may have been representing the actual sentiments of the silent majority of their constituents in opposition to the screams of a vocal minority..."
I first heard the term applied to my generation (1940s-1950s coming of age) in about 1960.
I recently purchased a dozen lapel pins which say:
Silent Majority ... No More
I put them on the table at the TEA Party rally beside the donation bucket. They were gone in less than an hour.
We are not as silent right now as we once were.