Shprocket, "open sesame" is widely thought to have been “inspired by the fact that the pods of the sesame plant burst open when the enclosed seeds reach maturity.” The name of the seed is over 4,000 years old and is “one of the few to have entered modern languages from the ancient Egyptian (sesemt).” (From MAGIC WORDS: A DICTIONARY)

Miss Cellania, Groucho Marx's show technically featured a "secret word," but it had magical consequences.

Good point, Scooter -- how you order and pronounce the magic word is crucial! Imagine saying the words "I love you" in the classic monotone of Ben Stein. That probably wouldn't manifest any wedding bells. Comedian Russell Brand related proposing to a woman who spoke only Spanish, and he clumsily told her he loved himself.

Molli, you've named three of the most famous magic words, to be sure. As to my motivation, this reviewer summed it up pretty well:

http://mysteryarts.blogspot.com/2009/01/review.html

Yes, MadMolecule, the sacred and profane go together like light and shadow. "As above, so below," as it's been said.