Also is allegedly the motto of the mysterious secret society called the Prieure de Sion, and of the Plantard family, which is very important in it. Someone pointed out that it's also an anagram for another Latin phrase whose meaning is relevant to what the society seems to stand for: "I Tego Arcana Dei." -> Begone! I Conceal the Secrets of God.
The phrase was not coined by Nicolas Poussin, although it certainly does appear in a few of his significant works. There is a painting by Giovanni Francesco Guercino, dated 1618, predating Poussin's work by more than two decades, that incorporates the phrase as well. Guercino's "Et In Arcadia Ego" actually does seem to be the inspiration for Poussin's "The Shepherds of Arcadia" with shepherds contemplating a tomb with the famous phrase. It seems that Guercino was just as mixed up in secret societies, Masonic traditions, and other esoteric lore as Poussin ever was, so no real surprise there. There seems to have been no known public use of the phrase before Guercino, although the motif of Arcadia intimately wound up with it is indeed ages old.